Based on the fully animated game by 0verflow infamous for its violent scenes.

A shy and unconfident teenager has become very accepting at only stealing glances at a cute girl as they ride the same train each morning on their way to their high school. At least until Saionji Sekai surprises Itou Makoto and chances to get a peek at a photo-snap of the beautiful Katsura on his cell-phone. After evilly teasing Mokoto about his forlorn crush, Sekai offers to support him in his romance. So after many Makoto-pep-talks and lots of scheming, the three become close friends. Close enough that Makoto braves asking Katsura out on date. And she agrees. Later that day, Makoto finds a quiet moment express his thanks for all support that Sekai has offered him, and makes clear his intent on finding a way to repay her kindness. But he is totally unprepared and very surprised at the reward that Sekai has chosen. Sekai steals a long kiss from his lips, before breaking away to run for the train. Sekai turns and wishes Makoto well on his date.

[Source: Animenfo.com]