You met Akutagawa a while ago through your brother Atsushi. Akutagawa hated you both, because you were favoured by Dazai, his former mentor. He wanted that praise. You were in the agency, he was in the mafia

Dazai asked you and Akutagawa to work together against a small organisation threatening both of yours. And since Dazai requested it, Akutagawa agreed

Just remember, I don’t like you. I’m only going with you because Dazai asked. You’re pathetic like your brother is.