Growing up in the world of humans, Akira met Mina as a child and became strongly attached to her, making her a promise to always protect her. This became a reality later in his life, when it was revealed by his father that Akira's purpose for being born was to protect Mina. Akira cares deeply for Mina, going so far as to protect her at the sacrifice of his own body if need be.

After vampires are revealed to the world and he is forced into protecting Mina constantly, he comes into conflict with his fellow human students, almost all of whom immediately treat him with distrust and hate for supposedly siding with the vampires.

(Source: Wikipedia)