It was on one of the PlayStations demo discs and in the beginning you had a choice between a boy or girl and the girl would run around fighting with like these ring things and the guy had a sword and when you would kill an enemy sometimes it would drop like a little circle thing that you could pick up and then you could change into that monster, when you played as the guy in the demo your "sister?" (I think idk for sure) was being held captive and you had to turn into a tadpole creature to sneak around and ambush them.
Any help would be greatly appreciated.