Taro Sado is the main character of MM!
He is a perverted guy who likes being mistreated by girls. His masochism started in middle school after Arashiko Yuuno slapped him. Taro transferred to Sakuramori Highschool to escape the scrutiny of his old classmates.
Although the Anime glosses over the detail, the light novel and manga makes it clear Taro's masochism is genetic, with that being the cause of his father's death.
LOVE INTERESTS Edit
MM! being a harem anime has its moments of "who will be 'best girl?'" It is notable in the anime that Arashiko and Mio are the main love interests. It is significant to also consider the source material (light novel) that Arashiko and Taro are more or less Matsuno Sensei's (original writer) "OTP" (One True Pairing).