A fanart of Shun with the Sign Stand effect equipped. Source The text says something like "Shut up!" (うるせえ) and "The sign stand thing" (立て看板ぶっけんぞ).
Shun (峻) = High, steep. Can be also read as "Takashi".
Older teen/young-adult (?), older than Katsuki.
Shun (峻) is one of the two playable characters of Again, Sometime. (また、いつか。).
Shun has a bit long, messy black hair with a few standing out strands. He wears a red tank top, dark blue jeans and brown sports shoes.
(Source: In-game profile)
Shun and the other playable character, Katsuki (克樹), have several effects in common. For example, Katsuki (being the younger one, has an effect that turns him into a high school student. In return, Shun can be turn with an effect into a middle-school student, looking more like a boy of Katsuki's age.
Both Katsuki and Shun refuse to leave their room/apartment. Both of them explore their dreams, are male, have short black hair, and are sentimental.
It is unclear what is the relation between Katsuki and Shun. Judging by the title, "Again, sometime", it could be that they were separated once, and they wish to meet again sometime.
They could be brothers, judging by their appearance. They could be (childhood) friends who were set apart by life circumstances. That could be implied by the fact that you switch between them using the similar telephones in each of their rooms, also hinting us that they are now connected by phone calls.
Judging by their similar appearance and that they are sharing the same mood status - sentimental, they even could be the same person. He can either be one who decided to change his name when he grew up, or one that has split personalities named Katsuki and Shun.