The Mimic, as the name implies, is a mimic. This mimic was one of the first ones to be cursed with mimicry.
It is trying to kill an ancient chest-mancer that started cursing chests in the Gungeon, which turned them into mimics. Instead of fighting back, they hid in the halls of the Gungeon, hoping to not be found and killed by gungeoneers. They want to return and fight for the lives of all chests, existing ones and ones that haven't been made yet.
It has special hearts that can only be healed with the power of one of the mimics passive items(Chest Heart.), the other item (Chest Tongue.) lets them open mimics, until a chest is destroyed. If so, mimics will them automatically aggravate and have a higher spawnrate. This item will be unlocked for all gungeoneers to use once the Mimic has beaten his past.
These passives don't relate to the lore, but relate to the fact that the chest is now a mimic for all eternity and even though it can't cure itself, it can save all the other chests from the curse. The starting weapon is the weapon that most mimics use, the triple-shot revolver.
EDIT: Mimic can't drop any items. It makes sense.
It shoots 3 bullets at once, and has 15 ammo in a clip. It has a 1 second reload time and does equal damage of that to the budget revolver.
It heals half a heart of the Mimics special hearts every time that it picks up an item or a gun.
The item gives the ability to open mimics without aggravating them. But if you ever break a chest in a run, that buff is removed and now automatically aggravates mimics, and increases their spawnrate. EDIT: Aggravated mimics do double damage.