Someone who claims to speak the truth

• One who frankly admits
• Solemn promiser
• Oath taker
• Oath taker, essentially
• Someone who admits or acknowledges openly and boldly
• One who openly declares
• One saying "I do"
• Deponent

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Avower \A*vow"er\, n. One who avows or asserts.