I thought spring episode would take place directly after winter and be from Satoshi's POV.
Tsugumi's sister (Saeko) would get furious against the PE teacher about what he did to her sister.
Satoshi and Saeko are both depressed and their common misfortune bring them closer.
Satoshi begins to develop feelings toward Saeko. Meanwhile, at school, Satoshi who became a delinquent due to his trauma, gets approached by a gyaru.
Then, there are 2 routes:
Route A (NTR): Date the gyaru. Satoshi finally starts to become happy and confident in himself. He then tragically realizes that she was faking it from the start and it was just a trap: The gyaru was sent by Kujo (Autumn bully) to keep you away from Saeko so he can gradually seduce her. After days, she becomes gradually corrupted to him and you receive a video where he bangs Saeko, the gyaru, Azusa and Tsugumi. Satoshi hangs himself.
Route B (NTR 2): You date Saeko, it's all good until one day, you surprise her getting rawed by her dog. (There were a lot of clues disseminated in the story, but you have to play from Saeko's POV to see them explicitely)