The Peace Sticker is simply a sticker with the peace symbol on it, a symbol of the hippie movement. As per normal flair standard in EBF4, male characters wear it on their chest, while females wear it either on their skirt, or on their hair if it is equipped in the third Flair slot.
In EBF4 it overrides its owner's weapon status effect with Weaken, and also provides light Tired, Curse and Weaken resistance when worn. In the case of multiple status-overriding flairs being worn together, only the one at the leftmost flair slot will have its effect applied to the weapon.
The Peace Sticker remains mostly the same in EBF5, except that now it no longer provides resistance to Curse, and also got a small Defense boost.