The term "contester" refers to someone who participates in a competition, challenge or dispute. Its antonyms can be categorized into different groups, such as:
1. Non-participant - this includes words like "spectator", "observer", and "bystander".
2. Passive participant - these are people who are involved but not actively competing, such as "supporter", "fan", and "follower".
3. Opponent - these are people who are actively competing against the person labeled as the contester. synonyms in this group include "foe", "rival", "competitor", and "challenger".
4. Collaborator - these are people who work together in a competition instead of competing against each other such as "teammate" or "ally".