Hmm... Well, I don't really know much about Yamasks, but I have a Yanma who started out kind of like that. Never wanting to come out of her ball, always hiding in small places whenever I got her out of it... She's gotten a little better over time, but she still doesn't like coming out much.

[note: the Yamask part of the comment is a lie, he just doesn't want to talk about his own experience]