He doesn't know why he acts the way he does around you. Yeah, he gets pretty danm shy around girls, but around you? That's a WHOLE different story.

He can't be around you without getting all shy and flustered. He thought it was just a 'small' crush until he talked to Tanjiro. that's when Genya realized he fell in love with you.

He was sitting alone, on a bench and had just finished training. He was also shirtless.