So, there isn't a whole lot I can say about this character quite yet because the series Barajou no Kiss hasn't gotten past the 17th chapter in English translation. In fact, the main directive of the series has just started.
Right off the bat though, this series about knights and roses isn't everything that it seems. Mutsuki, unlike the other knights who tend to be fairly outgoing tends to remain in the background. He's mentioned that he's a creature called a night stalker (which could perhaps be loosely associated with vampires). His initial view of his master, the Rose princess (one who can summon the knights and let them use their magic at the cost of some of her own blood) when she first appeared as Agnes was an immediate refusal to recognize her authority. He, however, being the most skilled at searching out things... and she looking for a necklace... made it completely inevitable for him to be summoned. He was of course quite moody, but he still followed her orders without fail. It seems to be that he takes pride in his abilities, as well as his duty. After she attempted to take the fall by almost getting caught in her search for the necklace, Mutsuki softens his demeanor towards her. He even uses his ability to fly to whisk her away from the problem area. When he sets her down and manges to accidentally see up her skirt--which she notices--and Agnes reprimands him for it, he doesn't retaliate.
Perhaps his situation with living in the church which is seemingly under control of another knight in the group may be the reason why he is so cold a lot of the time. It was once stated that Mutsuki was even the property of the church. As odd as that is, it doesn't appear to impede the way he goes about his duties in regard Agnes.
He calls humans "lower life forms," but as he is called away to help Agnes privately he seems to have developed a great deal of care towards her. When she cannot continue with the search, he covers her with his coat and flies her home. Even after he leaves to return home himself and is cornered, he takes a nearly life-threatening beating, rather than using his abilities and making Agnes weaker.
Later, after Mutsuki heals and another knight in the group is attacked, he agrees to fly Agnes home. He begins by saying the knights (as a group) would protect her, but halfway through, he stops and changes his wording, so that he (as an individual) would die for her. When one of the group disappears, he's also the first to find Agnes and offer his assistance to find the lost lamb.
There's also a time later when he seems to have let something slip through his fingers. Agnes watches him take time to be alone. She, wanting to talk to him, brings him a sandwich. Now, he claims that his hands were dirty, but rather than grabbing the sandwich to eat it, he grabs her hand and eats the sandwich from her hand. Then he proceeds to rub his face on her arm. She, shocked out of her wits, reprimands him, tells him to take the sandwich and eat it alone, and orders him to feel better. Then she leaves.
Now, given, this series is kind of like some of the other fantasy tales with the objective of forbidden fruit. Agnes's ultimate objective is to eventually come to a point where she's in such a relationship with one of the knights that constitutes her being able to make love to him, therefore making the summoning process completely painless. Her other option being a pain-staking quest of locating a whole bunch of fragments to finish repairing the Devil's seal.
As a character, Mutsuki is in fact the dark mysterious guy that all us girls have been referring to for all of our lives, I'm sure. He is in fact the color of the black rose, he likes darkness and is its master. There are things that he doesn't totally understand due to the difference between human and himself. He claims not to feel, but in fact, probably feels much deeper than any of the other three. Other than Kaede, Mutsuki is the only other knight ever summoned by Agnes and spent time with her alone. The things in his past that he can't recall probably make up for some of the gaps in his personality, but even as rough around the edges as he is, he never truly means any real harm.