Is there any pretty monsters that anybody knows, such as a nymph, sirens, and such?

Do you want to adopt one, draw one, what? Monster and pretty are not synonymous.

For a character of a story I'm writing.

It really all depends on how your story unfolds.

For example, is this monster supposed to become good?

NOT vampires. Heh, just kidding :-D

Shape shifters can be pretty, or at least they can appear pretty. Also, in Balkan mythology there is a woman-spirit called Samodiva, which is pretty close to the Syren but more exotic :-)

This monster can be either.
Thanks, Felicity6!