At the beginning of the series, he treated Akira as an enemy, but Akira still sees him as a friend. His treatment to Akira is due to the fact he didn't accept Yuki's feelings and that Akira is Mina's servant, in which Ryohei believes he is siding with the vampires. His hatred towards Vampires is so strong, that he agreed with students beating up Hikosaka because he "associated" with Mina. However, after chatting with Angel, he decided to apologize to Akira, which Akira accepts.

(Source: Wikipedia)