Hayato Shingu

He is a passionate person who constantly lives in the moment, and can switch from rage to shock to grief all in an instant. Hayato has a powerful sense of justice, and will instantly leap in to protect the helpless or punish evildoers. When Hayato was seven he saw Egrigori slaughter everyone in his home village and burn it to the ground. Keith Black, one of Keith White clones (collectively known as "The Keiths", who could survive an ARMS implantation), killed Hayato's parents before his eyes and cut off his left arm, which was later replaced by his ARMS, the White Knight. Hayato then went to live with his grandfather Juzo and learned his Shingu style of martial arts. Haunted by his parents' deaths Hayato only lived for revenge, and he hunted down and destroyed Egrigori agents wherever he found them. After Hayato meets Ryo and his fellow ARMS, he gains more self-control, self-awareness, and compassion, which allows him to unlock the 'Heart of Water' technique; a skill that allows him to read his opponents - shortly after, he used it to defeat the superhumanly strong mutant martial artist, Kou Karunagi. Eventually Hayato gives up his revenge and becomes a protector rather than a destroyer, allowing him to access the Knight's ultimate form. Hayato and Kei grow closer over the course of the series, and as of the epilogue, have apparently been in a relationship for some time.