The Joker

The Joker is a recurring playable character within the Batman: Arkham series, being a minor but still controllable character in:
 * Batman: Arkham Asylum
 * Batman: Arkham Origins
 * Batman: Arkham Knight 

Biography
The Joker's backstory and even real name are a complete mystery, but what is known is that he was an aspiring criminal who overthrew Gotham City mob boss Black Mask and his gang and became arch-enemies with Batman for a solid decade. The Joker's playable sections are in Batman: Arkham Origins, during a dream section where he beats up his inner demons in a brawl and in multiplayer where he leads his own gang against Bane and his gang, and Batman: Arkham Asylum where he can murder multiple Arkham security guards in challenge maps. The Joker is also playable in Batman: Arkham Knight as a subconscious apparition of Batman's, but this singular scene is more of an interactive cutscene rather than an example of gameplay.

Abilities and equipment
The Joker, like Batman, has his own small inventory to help him whilst escaping Arkham Asylum. These gadgets include a single-bullet handgun that instantly kills anyone in its sight (and can penetrate enemies to kill multiple in a one shot), a pair of cartoony X-Ray spectacles that lets the Joker see through walls like Batman's Detective Mode, and a pair of explosive, remote-controlled chattering teeth. The Joker is an extremely fast and vicious combatant, but due to not possessing any body-armor like Batman, is incredibly fragile and easy to kill.