Ranmaru is one of the possible love interests in the game.
In the anime, Haruka first meets him when he performs 'Poison Kiss' along with the rest of Quartet Night in Season 2. However, they don't really interact much until Season 3, when Haruka is trying to compose his individual song. Ranmaru tells her that he doesn't want a rock song, since he is giving up rock, which shocks everyone.
Later, when Ranmaru begins having a nightmare about his past, Haruka wakes him up and he tells her what happened. He then explains that in order to devote himself completely to QUARTET NIGHT, he must break free from his past, and since rock is such a huge part of it he must give it up. However, Haruka is able to get through to him when she composes his individual song, saying that he shouldn't have to give up rock if he loves it.
After this, Ranmaru seems to like Haruka, and agrees with Reiji when he lightly threatens STARISH with the possibility of them taking her from them.
In Season 4 Episode 1, after QUARTET NIGHT take Haruka away from HEAVENS, Ranmaru holds her close and says that he want her to "unleash her music with him." He is then shocked when STARISH pulls her away from QUARTET NIGHT.