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 Аниме 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 Друзья to solve the mystery of the murder together. You'll have to watch the Аниме if Ты want to know if they stay Друзья или go back to being enemies though :)