Assertor

·noun One who asserts or avers; one who maintains or vindicates a claim or a right; an affirmer, supporter, or vindicator; a defender; an Asserter.

assertor
assertor (ads-), ōris, m. [2. assero], one who formally asserts that another is free or a slave. I...

A New Latin Dictionary by Charlton T. Lewis Ph.D. and Charles Short, LL. D.
assertor
adsertor ōris, m 2 adsero, one who claims (as master): puellae, L.— A defender, advocate , O. ...

An Elementary Latin Dictionary