I somewhat understand what the author want to portray :
- Anjo feels safe around Seto because he's not trying to jump her. With her provocative attitude and bitchy look, she's seen as an easy "whorish" girl by the other people but she can be herself with Seto without him trying to make a move.
- She knows that Seto has a crush on her since he's nervous and easily flustered when she's around and it flatters her. Anjo even seems to consider this as "cool and manly" of him, since he doesn't act on it (see first point).
- She also think that Seto is a genuinely good guy since he would give his seat to an old lady, defend a damsel in distress and work hard for the sake of the upcoming school tournament.
Now... It doesn't work that well because Seto is not actually a "good guy" but simply a "nice guy" with big self-esteem issues. He didn't shield the girl from the molester out of goodwill, but simply to prove himself that he's not worthless. He didn't clean the toilet or make the pom-pom because he's selfless but because the tasks were pushed onto him by the other students who wanted to slack off. He doesn't make a move on Anjo because he's a complete push-over, not because he actually respects her (otherwise, he wouldn't have closed his eyes and accepted her "fox kiss" in chapter 1). He doesn't act on his impulses because he's lacking the confidence to do so, not because he has any kind of self-control (and Anjo should know it).
Just compare Seto with Gojou from "Sono Bisque Doll..." and you'll see what I mean when I try to make a dichotomy between a "nice guy" and a "good guy". Gojou is an actual good fellow trying hard to control himself, acting reasonably bashful and showing genuine respect and admiration toward the girl while Seto is none of that. And normally, girls are pretty good at making this distinction, too.
That's why it doesn't feel plausible to have Anjo fall in love with him. He's way too dorky to attract a girl like her. She either has hidden self-confidence issues or something happened behind the scene (ie. in the past) for it to make sense. But nothing has been hinted so far, there's no foreshadowing going this way, and it would somewhat be out-of-character for Anjo. In the end, it feels like the author didn't give her a good reason to grow attached to him.