[sgwc]

Until ep 6, I consider Junichi as someone who react differently towards 3 brackets of girls.

bracket 1, Senpai or dignified people [Haruka & Tsukasa]:
Junichi will act kinda shy. It is like he aims for Haruka/Tsukasa but somehow maintain a set of principles as to not piss Haruka/Tsukasa off. Often help Haruka/Tsukasa in their pleads or chores.

bracket 2, classmate or girls of the same age [Kaoru & Rihoko]:
Friendly and likes to tease Kaoru and Rihoko a lot with nasty remarks.

bracket 3, little sister [Miya] or kouhais [Sae & Ai]
He cherishes or adores them like a brother would. Example would be in ep 2 where Miya and Haruka met for the 2nd time. Miya runs away and Junichi chase after her instead of staying with Haruka. With Ai and Sae, Junichi will lead them in their relationship [mostly with Sae].

Please don't take this post as a fact. It's just based from my observation from ep 1 to ep 6 plus the 3 ongoing mangas.