Thanks for the reminder but unfortunaly, Kazuma is in love with Yui : he considers her as a girl and he is angry when Yuuta is careless with Yui's body. The problem about him is that, as Yuuta said to him, he should be with the person he likes but he's always too kind with others so everyone mistake who is his girlfriend. I guss that girl is the one who endlessly asking for help (ch23). Hikaru is just trying to not hurt Yui's feelings and A GIRLFRIEND! But if he knows the truth maybe he'll fight for her!
I also wonder what will happen if Tachibana falls for Yuuta in Yui's body. Kazuma, Yui and Hikaru would get jealous.
*and is asking for A GIRLFRIEND
OMG Hikaru knows Yui (as Yuuta) has a crush on Kazuma, and is so cute trying to not let her get hurt by the news of the GF:
http://www.mangago.zone/read-manga/shishunki_bitter_change/bt/324794/Vol4_Ch26/7/
http://www.mangago.zone/read-manga/shishunki_bitter_change/bt/324794/Vol4_Ch26/8/
And he was all supportive, even with them being his guy friends, talking about how Kazuma thinks of Yui (Yuuta) being important, and hinting that even if they got together he'd still be friends:
http://www.mangago.zone/read-manga/shishunki_bitter_change/bt/295111/Vol3_Ch22/23/
http://www.mangago.zone/read-manga/shishunki_bitter_change/bt/295111/Vol3_Ch22/24/
I'm warming up to the idea of him being a possible partner for Yui, especially if she returns to her own body. They'd be so cute together