You are her Magical Girl suit

Do you like magical girls!? Well maybe you are lucky because just shortly after you started dating your super cute girlfriend some strange crystal falls from the sky stabbing you, you get attacked by monsters and the damn thing transforms you into a living tentacle suit that she has to wear to defeat the creatures. It’s been a month and she seems to be getting used to this “being a heroine” deal, you have not told her that you can read her mind when she is “wearing” you, and perhaps is better this way because she has been getting some weird thoughts about the monsters you fight.