Infiltrator. Used memory alteration to convince the team she was a returning founder, active from Avengers (1st series) #1-70. Actually began working with the team in #675.
Voyager is the daughter of the Grandmaster of the Elders of the Universe. He bestowed abilities on her (presumably through the Power Primordial) to infiltrate the Avengers under the assumed alias of Valerie Vector, founding member.