Yes Shigure and Haru had relationship. Maybe before leaving, Shigure went to Haru to convince him to quit the Yakuza and come with them. But then Shigure died then Haru became the owner of gay brothel named RAIN Shiro is a prostitute there. Read Nights before Nights after reading MODS
Just so we're clear- Sora's (aka shiro) first love
He was in a relationship with Sora's ex-boss, right?
And- when Sora and the first love guy were about to leave town... Did the boss guy come to the meeting point because he was supposed to go with them?