A person with whom you may have a lot of fun and/or a lot in common, who also has a vile and random dark side. These relationships are worth doing a cost/benefit analysis on. Also, limiting relationships with frenemies to non-SO relationships is a must. Why? Because it’s ultimately all about them after all, and you will need to be in a position to get away from them for indefinite periods of time. Know too, that you will probably become their frenemy as well—because you won’t be able to keep from talking behind their back. If this happens, don‘t expect everyone to get it—some will wonder why this person enrages you so much and others will wonder what you see in them.
It is recommended for your own safety/sanity that you limit yourself to one frenemy at any given time in your life.