Thank you, guys.
I thought about possible OP too, but given that it takes so much time and effort to use this card (first, you have to summon it with 6 quanta > then, skill cannot be activated the turn you summon it, so you have to wait for 1 turn > then, you can activate Soul Stealer but you can't use the stolen skill yet b/c you just used your skill, so wait another turn > then, finally you can start using the stolen skill every turn (if you have enough quanta). So, I don't think it's OP.
By the way, just so it's clear, this card "steals" skill, not copy or gain. In other words, the targeted creature "loses" its skill after it is stolen.