Izuku with a quirk that turns him into a beast whenever his "princesses" (the girls he gives the rings his quirk makes to) are ether in danger or order him to use his powers (IF he accepts the order.)

After his power is used the princess(es) will have no choice but to reward him with at least a kiss (minimum difficulty) with the reward getting more sexy the harder the task, the more power used and/or the longer he is transformed.

Also if he uses 10% or more of his power he enters an advanced beast mode that is based on the girl or girls he is using his power for. His advanced forms will get more complex the more girls he is currently using his power for.

His basic beast forms will be the base form the main character of Slave of the Magic Capitals Elite Troops takes when he goes slave mode, just with the collar and chains replaced with a mane.