If you could please implement these 'friending' relationships ASAP, that would be just great.
Someone whom has a particular interest in defeating you and you in them. Symmetric. At most one.
Nemesis is now used as a term used to describe one's worst enemy. Sherlock Holmes and Professor Moriarty are examples of nemeses.
Someone who you are an enemy with. Often symmetric. Opposite of friend.
Someone who annoys you but not to the point of antagonism.
An evil twin is the concept in fiction of someone equal to a character in all respects, except for a radically inverted morality. Symmetric.
If the evil twin is literally a twin brother or sister, it should be combined with the XFN value of sibling.
The evil twin value can be applied to a version of yourself from an alternate universe or timeline. The Star Trek episode Mirror, Mirror contains the canonical example of an alternate universe evil twin. The presence of a goatee beard is often a dead giveaway.
Someone in the same field of study/activity with whom you are vying for recognition and/or advancement. Often symmetric.
Someone who makes you feel furious. Opposite of muse.
In Roman mythology, the furies were hideous snake-haired monsters from who pursued unpunished criminals.
People who infuriate you do not need to know you personally e.g. an incompetent president or a self-righteous rockstar.
Someone who instills a feeling of creepiness in you. Opposite of crush.