One of a group of people who work together
Colleague
A person who disagrees with something and speaks against it or tries to change it
Opponent
A person who shares an apartment or flat with another person
Flatmate / Roommate
Someone who is always there for you, who cares about your well-being
Close friend
A player on the same team
Teammate
A person who hates or opposes another person and tries to harm them or stop them from doing something
Enemy
A person who lives in the house next to someone
Next-door neighbour
A friend who travels with you
Travel buddy
Someone who is in the same class as you at school
Classmate
A person who you know well and who you like a lot, but who is usually not a member of your family
Best friend / mate
A member of your family
Relative
Someone you do not know
Stranger
A person, group, etc. competing with others for the same thing or in the same area
Rival
A person that you have met but do not know well
Acquaintance
A brother or sister
Sibling