Canary

A person who makes you less 'you'. Someone who can calm you down, make you less angry and destructive, who reminds you that there is still lots of good and love left in the world.

I. e. Hughie is Butcher's Canary.

Person 1: Dude, did you see how Garret went from almost beating that guy up to laughing about it and hugging Emma in like thirty seconds?

Person 2: Yeah, she's his canary.