He is an incredibly passionate person. He constantly lives in the moment, and can switch from rage to shock to grief all in an instant. He also 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. Hayato also saw Keith murder his father and slice off Hayato's own left arm. Hayato's left arm was replaced by his ARMS. Hayato then went to live with his grandfather Juzo and learned 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. Eventually Hayato gives up his revenge and becomes a protector rather than a destroyer.
Hayato and Kei seem to be forming a relationship as time passes. Whether this will develop into anything remains to be seen. Hayato's ARMS is called White Knight (or just Knight), and is concentrated in his left arm.
The Knight seems to be the manifestation of Alice's will to protect. Hayato's arm transforms into multiple fighting blades and a projectile launcher for close combat. The blades produced by the Knight are supposedly the hardest substance that an ARMS can produce, however this seems to be influenced to some degree by the emotional state and skill level of Hayato. The Knight's nano-machines allow Hayato to heal injuries much faster than a normal human even in his untransformed state. White Knight's ultimate form is a powerful armored knight whose lance, the "Lance of Mistilteinn," can destroy other ARMS with an Anti-ARMS Nano-Virus, and as such, it negates the regenerative property of ARMS.