Post characters who were enemies/adversaries one-time but now became friends/allies

Lyon and Gray used to be rivals but now they're not

Sebastian & Agni (Black Butler)

They were once enemies but then became friends.....

In K-project, the anime starts out by Kuroh trying to kill Shiro to bring him to justice for murder. However, Kuroh ends up deciding not to and they become allies and friends to solve the mystery of the murder together. You'll have to watch the anime if you want to know if they stay friends or go back to being enemies though :)