Glad I didn't drop this and even kept notes of the characters. The premise of the protagonist having to tutor someone older than him—and a rare adult gyaru (also a young widow at 23yo) at that—is actually pretty memorable. A college-aged onee-san tutoring a high school boy is a fairly common setup, but the other way around is not. This is the only story I know where a younger boy is tutoring an older girl.
How sweet of him to buy a cake for her. It's just a wholesome gesture of wanting to reward her hard work, and something I rarely expect in what seems to be an ecchi romcom on the outside.
Did he get the wrong apartment or... I guess this girl is Airi's younger sister(?) This new girl does not seem to be among the other girls I've listed before, like his busty classmate Haruka who tried and failed to tease him.