Love Song (YAMAGATA Satomi)

From Nakama:

Nakano Atsushi, first-year high-school student who didn't open his heart to anybody except his much-loved older sister, even while thinking of putting together a band, isolated himself. His older sister, worried for the obstinate Atsushi, got the cooperation of her fellow student Takase Yuuichi.
The sound of the guitar played by Takase, who takes part in an indie band, is in tune with Atsushi's heart and he was made prisoner by Takase's invite “I'd like you to sing together with me”, just like a love confession... With a lot of material drawn for the occasion, the wandering of a hazardous love.