I'm guess he was sexually assaulted because how Masaki lean over Kairi he had a reaction to Masaki shadow, so im also gonna guess Aoi is Kairi protective personality, so if something happens or triggers Kairi his adrenalin will boost up making Aoi come out. This also explain why Masaki isn't able make love to his lover(Kairi) & why Aoi said "its impossible for you do it with Kairi" because anything leading to sex can trigger Kairi trauma (if he was assault/raped)