Hello everyone I'm new here, 

I've been looking for a certain game ever since I was a kid. (I grew up on NES)

This is what I can remember:

You play a character that travels around.
You explore and fight enemies.
You vs bosses that once beaten, join your party.
I think the bosses are under a spell until you beat them.
Or possibly decide to join you because you beat them.

Any ideas on what it can be? I've been searching for years... 

It wasn't dark like Castlevania. And I think the characters had non proportional bodies. Like Rainbow Island or Mario. Similar in how the bosses are defeated then join you though! I also think there is dialogue before the boss fights begin. I don't think they change form once defeated either.

or maybe its a Ufouria the saga (u beat him and he join you) u can play with its character