Yotasuke Takahashi (高橋 世田介)

Takahashi Yotasuke, also known as Sekai-kun (セカイくん), is a former member of the Art cram school Yatora Yaguchi attended, and is a third-year high school student at Geidai.

Takahashi has a reserved and shy personality, but it is also shown to be standoffish and easy to anger. He’s blunt, and often says things as they are in his mind without stopping to explain to others what he means. He has complete confidence in his art skills and isn't afraid to admit it.

(Source: Blue Period Wiki)