The topic of Sebek’s attitude arises again during Fairy Gala IF, when Sebek immediately refuses to be of any assistance. Riddle comments that “his stubbornness is a headache even at club meetings.”
(Silver explains Sebek might have scared the diurnal fairies due to his nocturnal fae lineage, which Sebek himself likely also knew.)
Later, however, Sebek approaches the Fairy Gala team on his own, saying that if their mission doesn’t succeed then it will affect Malleus and Lilia. He vetoes Deuce and Epel’s ideas and lectures the students on the tastes of diurnal faeries, which inspires Ortho to come up with the idea they ultimately implement.
It seems there is a possibility that Sebek is interacting with other students socially, with Ace saying that he went to karaoke with Jack, Epel, Deuce, the prefect, Grim and Sebek (and that it was a disaster).
Ace has a voice line about talking with Sebek when Lilia appeared and gifted him a piece of candy. He teases Sebek during Fairy Gala IF and into getting him a birthday present.
The two overlap during Sebek birthday vignette where Sebek tells him about a time he went to see a heavy metal band perform at a small live house.
While it was not a band with which he was familiar the experience was recommended to him by Lilia, and he was certain to educate himself on the location’s rules and regulations as well as the methods and etiquette behind moshing.
Sebek says that there was only one small problem when one of the performers fell into the crowd, and as he refused to allow a show recommended to him by Lilia get canceled due to an injured performer, he helped the person back to the stage.
Ace points out that the person had likely stage dived, finding the situation so funny that he says he and Sebek should go to a live show together next time. Sebek refuses.
Twisted Wonderland Reference Note 