One of the MCs (usually female) develops a relationship with someone of the opposite sex who suddenly appears and who becomes a better friend and confidant than the MC's S.O., and with whom they spend more time being chummy. This sometimes leads to an affair, or even a breakup (see
360 Degrees for example), or the person just becomes an annoying, always constant 3rd wheel.
Oh, yeah, that was a pretty dirty trick to play on your boyfriend, Rin, having your new best friend help you spy on your boyfriend.