It's not the body I'm worried about actually, it's the mind. Imagine the entirity of your usual Harem archtypes all in one. That's like some super yan-tsun-dere-kuru-wtf haremite.
She's not really part of the harem. Then again, neither is Suu.
They technically are, but Lala is content to wait till he dies and Suu is perfectly fine with her lot in the house, she gets her sneaks in when she's either alone with him or with her out of house friends.