Of course the former "delinquent" would have high favorability to the only one who didn't treat him poorly, and now that he's TS'ed, that favorability has now became affection for her.
To be fair, she is a half-succubus who didn't already have their target wrapped around their finger (unlike Haruki, who'd claimed her Onii-chan's heart even before the TS). Gotta get Sensei to see her romantically, and lay her claim before anyone else can.
Well, Haruki's mom basically said in an earlier chapter that it was Haruki's strong feelings for her Onii-chan that triggered her half-succubus TS power, as Haruki was under the initial impression that he'd have to be a she to have a chance romantically (made clearer by one of their last focus chapters being a conversation between the two about her being afraid their relationship would end if she suddenly changed back to being a boy).
If there's any consistency in the lore, I'm betting that seeing Haruki do it (and be successful in her catch) triggered the little delinquent's desire to go over the edge of what it needed to activate her own half-succubus blood.