She wants an almost equal dynamic.

"Alright, count me in!" Setsuna grinned, standing up. "Here are my terms. Izuku, you and I will be totally equal partners as boyfriend and girlfriend, fair enough?"

"Fair enough," Izuku nodded. "And what about with all the other girls?"

"Well, they better get used to calling me Mistress if they're total subs," Setsuna smirked. "If not, then they will have to negotiate something in particular with me."

"I'm a switch, I'm submissive to guys, but prefer domming girls," Tooru chimed in. "We can be equal in our own dynamic if you like," she added.

"Fair enough. And you two?" she asked Momo and Ochaco.

"Total subs, Mistress~!" they chimed at once, before looking at each other, and giggling.

"Good!" Setsuna cackled as she rubbed her hands together. "This is gonna be fun!" She then looked at Izuku. "Who else is involved?"

"Mina and Kyoka," Izuku replied. "Though they are both dating outside of the harem. Kirishima for Mina, and Kaminari for Kyoka. I'm not bi, so no sexual relatioships with the guys for me, but the girls all have the option for having fun with them. Kirishima is a sub, Kaminari a dom, Mina is like Tooru, kind of switchy, but it's not gender dependent with her."

Setsuna nodded, and then looked at Izuku. "So... seal it with a kiss?" she flirted.

He smiled and leaned in, and she met him halfway, their lips meeting as she used her hands to grope his ass, and he returned the favor.