Multiple realities
(covers information from several alternate timelines)
For additional meanings of "Wallace", please see Wallace.

Wallace was the maiden name of the wife of Alexander Marcus. She was the mother of Carol Marcus and the grandmother of David Marcus. (Star Trek II: The Wrath of Khan)

In 2259 of the alternate reality, Carol Marcus used the alias "Carol Wallace", after her mother's maiden name, to board the USS Enterprise so no one would connect her with her father, Alexander Marcus. Spock confronted her with the truth of her deception in the weapons bay. (Star Trek Into Darkness)

This character was only mentioned in dialogue.
A deleted scene would have established her as having lived in England, where she raised Carol Marcus, thus explaining Carol's English accent. [1]
The Star Trek app for smartphones and tablets revealed her first name to be "June (β)".