A "frenemy" is someone you like overall, but also happen to be at odds with. These types of rivals might seem like a burden, but they're actually great to have around at your job.
This video from the New York Magazine YouTube channel explains why it's actually good to have an ambivalent relationship with some of the people you work with. It might seem like they're constantly stressing you out, but in reality, they're probably pushing you to do your job better. The unspoken competition between the two of you keeps you on your toes and refuses to let you grow complacent. You might complain about them from time to time, but you'll also be learning from them.