You have to use the location they give you when you buy the skeleton and try different mobs with the pumpkin on they head and make them cum
Spoilers for anyone who is trying to figure out how to get the characters without help:
Too get the new characters you have to try the pumpkin head on enderwoman, bee and so on (haven't figured out the rest yet) and you wait a few seconds or you use the piston with the said character with pumpkin on head. Here is proof of my progress.