frenemy

1. Someone who is ostensibly one's friend but who actually behaves in an antagonistic or treacherous manner. I let myself fall victim to the abusive behavior of too many frenemies in high school. I've learned since then only to hang out with people who actually value my company. If Tiffany started that nasty rumor about you, she's a frenemy, not a true friend. A: "It's ridiculous! Who sabotages a friend like that?" B: "A frenemy, that's who."

2. Someone who is one's friend but also acts as one's rival. A: "What's up with you and Jake? Do you guys dislike each other, or something?" B: "Nah, we're just frenemies. We goad each other on for the sake of competition." Yeah, I like the other catcher, but when you're competing for the same position on the team, you're also kind of frenemies. It's just what happens. We're not really frenemies. I know Jane is going to win most of the academic awards for our grade, and I'm OK with that.