On the V1 you just have to take bad care of it!
Often leave Happy & Hungry hearts empty for a few hours (but not so that it dies), don't get Training Points, leave a poo next to it for a long time without clearing up - or even let it go to sleep with one next to its bed all night....
Not sure how you get one with a V2, but can't see the harm in testing out the same strategy and see which character you end up with!
Best, etc.
TamaMum