A kendo prodigy and nicknamed the "White Devil" of kendo. Kendo has always run in his family and he's been practicing it for a long time. He ranked at the top of every national game he took part in, thus earning his title for his speed and his strength.

However, up to middle school he didn't really have any friends. All his time and energy was put into kendo. That all changed when he met Kei, who fell immediately in love with him. At first he was taken aback at Kei's straight-forwardness and thoroughly annoyed and disgusted with him. But he was also impressed with him and his rusty kendo skills. He invited Kei back to his house to practice and afterwards they shared a kiss. Though he didn't admit it at first it left quite an impression on him.

In high school he was captain of the kendo team. Kei and him ended up confessing their feelings for one another and took a physical level to their relationship. However, not too long later when he entered the kendo championships Kei learned his mother died. Seeing his lover's distress he vowed to always protect him. He ended up keeping his promise in a big way whena a yakuza hit man tried to run over Kei with a car and Ranmaru pushed him out of the way. The assault left Ranmaru paralyzed on his right side and believing he could never do his beloved kendo again. But after undergoing extensive physical therapy with Kei at his side, he managed to get his limbs moving again with a promising outlook on kendo again.

Ranmaru is completely shy with his feelings, but there is no denying he loves Kei. He's only been able to say the words 'I love you' to Kei once, but luckily Kei understands Ranmaru enough to know how he feels. Like Kei, Ranmaru wants no part of the yakuza life that nearly killed him, though as with Kei the yakuza life forces its way into Ranmaru's. However he takes kindly to Kai though he tries to draw the line when it comes to the younger man's affections. He wants Kei and Kai to have a brotherly relationship without the two of them having such strong animosity towards one another.