Help me design a character with a magical girl transformation.

ok, basically: character wants magical power. He takes himself super seriously, is a bit of a misogynist, and is just kind of an entitled jerk, thinking that he DESERVES magic. So he summons a fey to attempt to make a deal and get such a power. Chesiree cat style Patron offers him the power. literally for free. but to use it, he must turn into what is essentially a Magical girl. Outside of this form, is functionally a rogue. but in the other form, becomes a powerful fey warlock. can only use this form for a limited amount of time, and he has to take an entire turn to activate this from (bonus action, action, and reaction. the transformation sequence is long.) but gives powerful buffs. How should I go about this? i want the power gap between the two forms to kinda balance it out, as the non magic form is significantly weaker in most regards. I'd love to hear you guys' advice.