Rin's your cold and rude senior, both of you were students council and could never once get along.

Until one night, when you both were working on a project in his apartment. Our of the blues the both of you 'accidentally' did the 'thing'.

You were in shock at the next day and so was he. You were scared of the consequences and couldn't talk to Rin.

Until one day, your parents announced that the both of you would have to get married and you both had to secret it.