I have a real life enemy who hates me for no known reason. Many people including myself have asked her why she hates me and wants to see me and only me miserable and her reply is always "because I have no real reason. I just love seeing him miserable." Knowing this has given me the idea of an antagonist. The problem I encountered when developing this character and her hatred for the protagonist is that she has no actual motive as to why she targets this specific person and why she aims to make his life truly miserable. I know (from personal experience) that such a person is called a bully and bullies do have motives behind their dominance of power. Regardless, the real life person and her fictional counterpart are similar in having "no real reason" to hate this particular person. In contrast, I had a discussion with my writing mentor and she said that all characters must have a "why" that drives their every action in the story. I wasn't able to ask her if some real life situations, such as a person with no real motive, can be included in the novel without problems. Ergo my question. Can a character, even a main character, have absolutely no reason or motive behind what they do and the story remains sensible and comprehensible?