Acquaintance
noun.someone you know, but who is not a close friend.
Days turned to weeks and weeks into months. Working hand-in-hand got them closer. They would hang out at the end of the week with co-workers, share relationship issues once in a while, even visit each other's house for a sleepover. But they wouldn't classify themselves as friends. She was his mentor. He was her subordinate. To have anything more than that would be treading dangerous grounds.
Sometimes they would be pushed towards each other. But he would have a girlfriend when she's single. And she would be in a relationship when he's not. Whatever circumstance they were in, they knew it would be a bad idea to give in to what others think is best. Being in a relationship with a co-worker is a definite no.
