Zero is generally a composed person he is also protective of Yuki. Before the second 'part' he showed human Yuki his emotions and did not usually act hostile, and cold hearted, as he usually seemed to be towards the other Cross academy students. When Yuki was revealed to be a Pureblood vampire, he became cold and hostile, especially towards Kaname and Yuki. Though he appears otherwise, he is a gentle and kind person. Due to his childhood, he builds barriers around himself, rather than opening up to Yuki when they first got to know each other, a trait that is still evident even in the latest chapters of the manga. Yet despite this, he has a kind heart which longs to protect humans, he will never betray the ones he holds dear.
As a child, Zero was once a quiet and gentle boy, to the point that Ichiru questioned whether or not he was even capable of being a vampire hunter. However, after Shizuka's attack, Zero developed a strong hatred towards vampires. Zero is a tsundere character; he sports a scary aura that makes most of the Day Class students wary of him. He can also glare in a way that even Kaname Kuran calls scary. However, his innate nature is to protect humans, and his cold attitude towards those he loves is likely to push them away and keep everyone at a distance.