Blake's Gengar, Fade, is a bit of an odd case. He makes himself look all creepy and crawly, but it seems that if he gets to know you he'll occasionally help you with something, or some other nice gesture.
Sometimes Nico will let his former trainer keep watch over his children, if he needs to be somewhere overnight for example. Taffy ended up sleeping over on an unfortunate night where she had a nightmare. Fade barely knows her, but his past efforts in scaring her ended up in her giggling it off, confusing him and making him wonder if she's scared of anything.
Fade has the move Dream Eater, which can be used as an offensive move in battle... Really, that only works with good dreams - nightmares are not painful to eat for the target. Seeing the poor little shaymin whining in fear, crying for help... well. He couldn't just let her suffer!