Goodbye Trevor is a written note in Dishonored 2.
Anyways, I'm a Howler now and I ain't going back to my life like it was before. And if you don't want to join up too, because you're too scared or too weak, then to the Void with you! I already got four new boyfriends and a couple of girlfriends too, if I want 'em.
I'm done crying about it. So I guess this is goodbye – forever.
Unless you change your mind and join up.
This note can be looted from Trevor, in the past, during the mission A Crack in the Slab.