Blue acts in a completely emotionless and somewhat robotic manner, contrasting with Psyche’s behavior. He is also bad at interacting with others and prefers to be on his own. Some describe him as being somewhat of a kuudere, someone who is cynical and cold on the outside, not showing any care for others, but that deep down is actually someone who is nice and caring.
Blue has the opposite colors of Psyche, wearing mostly black with some blue details, like the fur on his coat and his headphones. He has blue eyes and his hair is white, but sometimes he seen with black.
Kaikyo originally found Blue online and started bothering him when he realized the other man was a hacker. Their relationship started with him pushing Blue around and taking advantage of Blue's emotionless behavior to boss him around, but before he realized it, Kaikyo was longing for any demonstration of emotion from Blue and soon he grew to love the smaller man and started wanting to protect him and show him that the world outside of his house and his computers was worth exploring.
Izaya hires Blue sometimes in search for information. This becomes their connection which and allows a relationship to begin, with Izaya being curious towards Blue and going to him in person.