As Arisa's boyfriend, he is shown to be faithful, loyal and cherishes Arisa very much. He is kind and displays significant physical affection towards "Arisa" - much to her unpreparedness.
When he sees "Arisa" hanging around Akira Manabe often, after her apparent recovery - he becomes obviously jealous and feels betrayed by her. This devastates Tsubasa and the two if them do not interact with each other for some time.
On the contrary, the truth is unravelled much later to reveal that Midori is the King who has indoctrinated Class 2-B with his wicked actions, power and presence.
As soon as that is unveiled, he is immediately portrayed to look significantly sinister, callous and sadistic - especially when he talks about or is with Tsubasa.
When the transfer student, Rei Kudou, his representative - reports to him that the real Arisa has finally awakened, Midori's usual evil expression wanes into sincere surprise - showing that although his general demeanor was a lie, his devotion to his girlfriend never was.
Together with Arisa, the pair are truly the King of Class 2-B and he want to completely eradicate Arisa's sister because of what she once told him.
In his memory, Arisa confessed to Midori that she was afraid of Tsubasa replacing her - since her mother favoured Tsubasa and talked to her often on the telephone.
Not long after the beginning of the story, the King knew that "Arisa" is actually Tsubasa dressed as her twin sister and attending Himetsubaki Junior High to investigate the environment that Arisa was in, to find out what made her attempt to commit suicide.
Therefore, Midori is actually a skilled liar and actor, cunning, manipulative and an elusive character that even goes as far as kissing his girlfriend's sister to convince Tsubasa with more ease and succeed in achieving his vile intentions.