~vs~
Nero is as he appears in Devil May Cry 4 in his base form. He has all his weapons and techniques gained through the game, and he may access Devil Trigger for 10 seconds every 5 minutes. The regen on his DT is as it's done in the gameplay though, not cutscenes.
Master Chief is as he appears by the end of Halo 3. He is equipped with his SPARTAN-II armor and rechargeable energy shield. He brings a Shotgun, Energy Sword, Assault Rifle, Battle Rifle, Rocket Launcher, Missile Pod, Needler, dual Brute Maulers, and Brute Shot with him into battle. He also has a single Deployable Cover. And yes, as long as they don't contradict the games, the novels are valid.
Samus is as she appears in Metroid Prime 3: Corruption, in her normal state near the end of the game. She may enter her Corrupted state for 14 seconds every 5 minutes with no ill effect. If she lasts for any longer than that in Hyper Mode she will die in the next 5 seconds. She's aware of this limit.
Fou-Lu is as he appears in his playable form in Breath of Fire IV. He has access to most skills and magic from his playable hybrid forms and can transform into any of his playable non-Tyrant/Infini hybrid forms, though he cannot use breath attacks.
The Prince is as he appears in the Prince of Persia series. The Time powers he has are those from Warriors Within, and he has 10 non-refillable sand slots. He may freely turn into and from the Dark Prince, as seen in The Two Thrones, and will not suffer from its life-draining effect. He also has all the powers gained from The Forgotten Sand (PS3/360 versions) save for The Power of Time and Memory.
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boring and mundane signature
Now with more bold! Less Italics due to bold