Definitions:

1. Referring to someone who provides testimony or evidence to support the truth or validity of something, typically through their personal knowledge, experience, or observation. NOUN
   • The attestor's detailed account of the accident played a crucial role in determining liability.
   • The lawyer called upon several attestors to corroborate the defendant's alibi.

Synonyms:

• affirmer
• certifier
• corroborator
• endorser
• substantiator
• supporter
• testifier
• verifier
• voucher
• witness