1. companion (someone with whom one spends time or keeps company)

2. mate, colleague; (with a qualifier) fellow X, classmate, coworker, etc. (someone with whom something is shared)

3. fellow member of a political group; comrade (politics)

4. teammate (one who is on the same team)

5. mate, partner (matching item in a set)

6. partner, significant other (especially not a spouse)

7. companion (a thing or phenomenon that is closely associated with another thing, phenomenon, or person) (figurative, figuratively)

8. friendly term of address; friend, buddy