Ken's sister Ringo is very well received by the fans. The 5th episode and 6th episode where she made an appearance received above 80% positive response, currently the show peak record.
Genius Bonus: One Imagine Spot shows a group of boys going ga-ga over Minatsu teaching math. The writing on the blackboard behind her is a proof of Euler's identity, "the most beautiful theorem in mathematics."
Les Yay: Several instances between Kurimu and Chizuru, like in episode two when Chizuru turns Kurimu into her pet, mindslave thing.
The Bittersweet Ending of the second season is both this and a Crowning Moment of Heartwarming. The girls of the student council all tell Ken that they love him (in unison!), but Kurimu and Chizuru are graduating while Minatsu and Mafuyu are transferring, leaving Ken to complete his last year at Hekiyou by himself.