What do you call a person who is not interested in relationship?

Aromantic is the word that describes, a person who has no interest in or desire for romantic relationships.

What do you call a person who doesnt like to socialize?

A loner is a person who does not seek out, or may actively avoid, interaction with other people. More than one type of loner exists, and those who meet the criteria for being called loners often actually enjoy social interactions with people but display a degree of introversion which leads them to seek out time alone.