Because his Dad probably forced him into heists and assassination jobs with him. Mercury probably just wanted someone to tell him he was doing a good job for once in his life.
rwby mercury black | Tumblr
RWBY mercury black
Very funny :)
Mercury by SidusRie.deviantart.com on @DeviantArt
rwby, emerald, mercury
RWBY - Mercury Black
Coco one of the most badass characters ever showed in the RWBY universe