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Name - Eiji Hino
Fandom - (Kamen Rider OOO (pronounced "ozu" or "O's")
Personality -
Eiji Hino comes across as a modern-day RPG hero, a wandering vagrant with a perpetual goofy grin on his face, someone who always has the time to listen to your problems and leaps at any chance to Right Wrongs. Most of the time, what you see is absolutely what you get: He's a cheerful guy with a ton of playful energy and an instant empathy for everyone he meets, and so has an easy time making friends wherever he goes. He takes his grandfather's advice, that all a man really needs is a pair of underpants for the next day, to heart -- both figuratively ("don't worry too hard about earthly possessions, so long as you have the basic necessities you'll be basically OK") and literally (he has a collection of brightly-coloured boxers. In all different printed patterns. Which he gives out to people like fortune cookies). When presented with the opportunity for adventure or just some odd scenario, his reaction is along the lines of, "Uh... I don't know what's going on, but... sure, let's go!"
At the same time, like many RPG heroes, the reason why Eiji has voluntarily become a homeless drifter puts things in a different light. He has PTSD from witnessing a civil war first-hand, he's become completely alienated from his blood family, and his disillusionment at his father's selfish actions has made Eiji reject all material desires, period. Also, the part about making friends? Up until very recently, this wasn't really true. Eiji has people he likes and who are grateful to him for his assistance, but there are very few individuals he really depends on, and used to retain no long-term attachments or relationships once he left a place behind.
Eiji's guilt over seeing a small child die (and his indirect responsibility for the circumstances that killed her) led to his promise never to let it happen again. In his own words, "If I don't reach out my hand when I can, the regret will make me wish I were dead." While he always wanted to travel and help people, it's become unhealthy and downright pathological, frequently throwing himself in lethal situations for the sake of total strangers, with no regard to the danger. Eiji lives from moment to moment, with no plans or regard for his own future, so long as other people can be safe and happy.
Hidden under the cheerful face is a surprisingly cunning streak that thinks fast under pressure, being able to suss Ankh out and keep him in line shortly after their initial meeting, and tricking Belle into wrecking her sadistic choice game. This is not to say that the naive goof is all an act, more that he's aware of how he appears to other people and is clever enough to use it to his advantage, when needs must. In spite of his humble-looking state, Eiji was raised a politician's son in a rich family, and he still views the world through the lens of someone separated from the common man. Combine this with his compulsive need to help everyone he sees, and you have someone who truly believes that it's all up to him, and only he can do it, and given the chance he'll seize any amount of power for a shot at being a saviour... and, given his intelligence, he's aware of how messed up this all is.
In short, a self-loathing headcase on a major power trip who still somehow manages to be a starry-eyed hero.
That said, at this canon point Eiji has made major headway in working through his past, such as acknowledging that despite all the awful things he's seen and experienced, he isn't alone and doesn't have to bear them on his own. Instead of having no ambition or familial ties, he knows now that he can reach out to the friends he sees as his new family, and he holds out hope that one day, he'll be able to revive Ankh.
tl;dr

History -
Eiji on Kamen Rider Wikia
Eiji on Wikipedia
(The above links mostly summarize the main OOO series, with very brief notes on the events in Megamax, which is why this section focuses on the latter.)
Eiji's current canon point is at the end of Movie War Megamax. This takes place a short time after the series, likely not longer than a few months later. Rather than entirely returning to his world-wandering ways, Eiji has been keeping in touch via video chat with his friends in Tokyo, as well as working alongside the Kougami Foundation (a powerful and mysterious company, whose chairman is the descendant of the original OOO) to develop new Core Medals, in the hopes of resurrecting Ankh. At that time, all the existing Core Medals had apparently been destroyed in Eiji's final confrontation with Dr. Kiyoto Maki, a man who had attempted to manipulate Eiji into becoming a Greeed, follow in the footsteps of the original OOO, and become a god-king to destroy the world.
In truth, a vortex had transported all the medals 40 years into the future, possessing the body of Michal Minato and transforming the boy from Kamen Rider Aqua into the Greeed Poseidon. Poseidon would use a time anomaly caused by a meteorite strike to travel to the present day and cause havoc. Along with a version of Ankh from the future, Eiji managed to subdue Poseidon, retrieve the Core Medals, and help Michal overcome his phobia of water -- soon followed by the Foundation X scientist Lem Kannagi stealing the Poseidon Medals.
Foundation X is a research organization ultimately behind several events in Kamen Rider W, having sponsored the Gaia Impact project and being responsible for the origin of NEVER. In here, they had been monitoring activities involving supernatural happenings, such as the O Medals and SOLU, an alien life form that had been on the meteorite which created the time anomaly. In this case, Lem Kannagi was stealing the Foundation's resources for his own ends, converting SOLU into an Astroswitch, a device fueled by cosmic energy. With the SOLU Switch and Poseidon's Medals, he could transform into the Super Galaxy King and take over the world.
In pursuit of Kannagi, Eiji would enlist the help of Gentaro Kisaragi (Kamen Rider Fourze), who had befriended SOLU before she had been kidnapped and turned into an Astroswitch, as well as Shotaro Hidari and Philip (Kamen Rider Double), who, along with their past with the Foundation, happened to run afoul of a convoy transporting SOLU. With Shotaro and Philip holding off some of Foundation X's forces, and Kamen Rider Aqua returning from the future to give Eiji the Super TaToBa medals as a gesture of thanks, Eiji and Gentaro chased Kannagi to the Exodus spaceship and took him down.
A word on the films, and Eiji's crossover encounters:
In the Kamen Rider franchise, the canonicity of the films are often... dubious at best. I at least consider Megamax canon since it acts as an epilogue to the main OOO series, tying up some of the loose ends of the finale, which would mean he's fought alongside Double and Fourze. He's appeared in a different film with Den-O, although that was a time-travel plot so it's all weird and wibbly-wobbly.
Additionally, he's run into the Seven Legendary Riders, also under weird circumstances -- Lem Kannagi had a machine that turned them into Medals and Switches, and Eiji and Gentaro summoned them back as humans. It's safe to say that the Legendary Riders, as the title suggests, have become figures of myth in the current day, regardless of whether they really existed in Eiji's timeline.
Aside from Megamax, the films with crossover appearances (eg. The Gaia Memories of Fate, Movie War Core, The Shogun and the 21 Core Medals, etc.) generally took place at some unspecified time mid-series.
Skills/Abilities/Powers -
Non-supernatural: Eiji is an accomplished world traveller and has all kinds of miscellaneous skills, not only related to travel but to various odd jobs he's done; he likely has a grasp of various languages alongside his native Japanese. While not extraordinarily powerful, in the Megamax film he's become significantly tougher and faster in melee combat, capable of taking out plenty of enemy mooks barehanded, to make up for the period of time when he couldn't transform.
Supernatural: With the OOO Driver, Eiji has the ability to transform into Kamen Rider OOO, an armored fighter with an intuitive mastery of close-range combat, swordplay, and a wide variety of powers. (While "OOO" is usually written as three capital-O's, it's more correctly represented as an infinity symbol with an extra O-shape, indicating that Kamen Rider OOO is a step beyond infinity.) To transform, he inserts three different Core Medals into the belt and activates them with the O Scanner device. The Scanner is also required to initiate a finishing move if Eiji is using a particular medal combination. The Driver belt is tied to Eiji, in that only he can use it, though anyone can help him use the O Scanner.
Incidentally, the Driver belt sings whenever Eiji transforms or swaps out medals. Really loudly. This is not an ability built for stealth.
The O Medals are large coins that are central to OOO's abilities, and are divided into Core and Cell Medals. Core Medals are sorted by colour and associated animal, and can either be used in a random combo, with each Medal affecting a segment of OOO's armor (Head, Arms, Legs), or three of the same colour to create a Full Combo with specialized abilities. Using a Full Combo is physically and mentally taxing on Eiji, although at this point in canon he's become inured to it.
There is an exhaustive list of what each O Medal can do at this wikia link, but a for very quick rundown of them:
Red/Bird (Taka, Kujaku, Condol)
TaJaDol/Blazing Combo. Capable of flight, seeing long distances, firing energy projectiles, and fire-based attacks; utilizes a flying drop kick as its finisher. More physically draining to use than the other combos, aside from GataKiriBa.
Yellow/Large Cat (Lion, Tora, Cheetah)
LaToraTah/Scorching Combo. Increased running speed and rapid kick attacks, fights with the Tora Claw gauntlets, and can release intense levels of heat and light to incinerate/blind enemies. Its finisher ability has Eiji charge at an opponent to deliver a cross-slash.
Green/Insect (Kuwagata, Kamakiri, Bata)
GataKiriBa/Strongest Combo. The horns on the helmet generate electricity, and the gauntlets gain bladed spurs. Increased jump height and kick strength. The most ridiculously exploitable form, due to its special ability to create temporary duplicates of himself where each copy can change their individual medal loadout, in one instance creating an entire team of different Full Combos. This has the downside of leaving Eiji seriously discombobulated and exhausted.
Silver/Heavy Mammal (Sai, Gorilla, Zou)
SaGohZo/Gravity Combo. Gains an increase in strength and gravity manipulation. When the medals are used individually, he fights with headbutts and using his gauntlets as fired projectiles, and can stomp on the ground to cause earthquakes (generally opening the ground under an opponent to trap them from the waist-down). In the Full Combo, he loses a lot of speed and mobility, and is usually limited to the gravity manipulation and stomps.
Blue/Aquatic Animal (Shachi, Unagi, Tako)
ShaUTa/Marine Combo. Allows Eiji to breathe underwater, shoot jets of water, emit sonic waves, turn his arms into eel-like whips to shock an opponent, and turn his legs into octopus tentacles that can cling to walls.
There is also the Multi Combo that utilizes the Taka, Tora and Batta medals, better known as the TaToBa Combo. This is Eiji's standard combination and most iconic one. It has the high jump from Batta, the claws from Tora, and improved sight and accuracy from Taka. In addition, Eiji possesses a set of Super Core Medals from the future, Super Taka, Super Tora, and Super Batta, which create the Super TaToBa, a powered-up version of TaToBa -- improved kicking strength, longer claws, flight, etc. The most remarkable aspect of it is the ability to briefly freeze an opponent in time to outmaneuver them.
In addition to the above, there exist purple medals (Dinosaur: Ptera, Torikera, Tyranno) and orange (Reptile: Cobra, Kame, Wani), but the purple medals have been destroyed and the orange ones aren't currently in his possession. As of Eiji's current canon point, he has one copy of each medal, aside from purple and orange. He has an additional broken Taka medal, which cannot be used to transform.
There are dozens of potential random combos of medals and their respective abilities, but honestly, he's going to stick with TaToBa for standard fighting and a Full Combo for unusual circumstances.
Cell Medals are silvery coins that are dropped by Yummies, creatures that are concentrated manifestations of human desires; the more Cell Medals a Yummy contains, the more powerful the Yummy is. Greeeds use them to restore energy and become more powerful, and Eiji uses them to power the Medajalibur sword. When used in combat, the sword is capable of cutting through almost anything -- its finishing move creates the illusion of slashing through the landscape itself, although any collateral damage apart from the sword's intended target auto-repairs when the attack completes. Cell Medals are most likely inaccessible in this world (unless a Greeed character is apped and they create Yummies out of people), which means Eiji won't be able to use the Medajalibur's supernatural abilities, although it should still work as an bladed weapon.
In Eiji's world there are vending machines called Ride Vendors, which accept Cell Medals and dispense little support robots called Candroids don't you love these stupid puns I sure do. Any Ride Vendor can also be turned into a motorcycle, which can be used by anyone. Since Ride Vendors are probably inaccessible in this world, Eiji won't have support robots and he will have to get a bike through ordinary means.
Power Restrictions -
Theoretically, if Eiji scans all the Core Medals at once, he could become a living god and destroy the world. The first Kamen Rider OOO attempted this with 30 medals; since Eiji has fewer than that, this scenario just can't happen ingame. If he does attempt this, I'd say that he'd go berserk, overload himself very quickly, and then do a moderate-to-high amount of damage to the area and people around him before passing out.
For nerfs to specific Combos: GataKiriBa will not have the ability to create duplicates that can use different combos. Super TaToBa cannot stop time, and is only a standard power upgrade from TaToBa.
Also, at the beginning of the series, he could not use a Full Combo without completely exhausting himself, and using different Full Combos in quick succession was really bad for his health. At this canon point he's become tough enough to withstand this, but to balance him out ingame he could revert to this prior state.
If necessary, he could lose all of his medals on entry except for the ones necessary for a TaToBa Combo, and slowly re-collect them as plot demands.
Notes
Along with the other core medals, Eiji is carrying a broken red medal that contains the consciousness and personality of his friend Ankh -- the series finale shows that Ankh is following him around as an invisible ghost. Should he lose this? On one hand, this is basically another whole character who exists independently of Eiji and could theoretically be revived if the red medal is repaired. On the other hand, this ghost can't interact or communicate with Eiji or anything in the physical world, and for all we know may not be entirely conscious or exist in any way beyond symbolic. It should be noted that when Eiji briefly meets Ankh from the future, the ghost is not brought up or addressed by the latter.
If Eiji loses the broken medal, he will probably devote all his time to trying to find it again, and even if he gets over it it'll colour the rest of his interactions with the setting; if he keeps it, with the OOC stipulation that it can't be fixed, he will probably relax on his quest to repair it, since he knows that at some point in the next few decades he'll succeed.