Tess is Mike's childhood friend and ex-girlfriend.
Background
Mike and Tess have known each other since they were 7. They have become very close since then and, eventually, became romantically involved with each other. They have also decided then that they would break up at the end of summer before college because Tess will be going away, but Tess never showed up to their last meeting, fearing that she might not be able to say goodbye.
Mike Ross, emotionally overwhelmed by the recent death of his grandma and being rightfully chastised by Harvey, gets back into smoking weed. In addition, after being told by Tess after a kiss that she is married, calls Tess and asks her to come over. Mike and Tess have an affair until Rachel stumbles upon it by coming by Mike's apartment unannounced. As a result of this and being rightfully chastised by Harvey again, Mike cuts ties with Tess. She isn't seen again.