Alexander Marcus was a Human Starfleet officer who lived in the 23rd century. He was married to a woman whose maiden name was Wallace. He was the father of Carol Marcus and the grandfather of David Marcus. (Star Trek II: The Wrath of Khan; Star Trek Into Darkness)
Alexander Marcus was only seen onscreen in Star Trek Into Darkness, which is set in the alternate reality. Memory Alpha assumes Carol Marcus' parents are the same in the prime reality because the timeline only diverged after the arrival of Nero, so the only things known about this version of Alexander are his familial connections.
The Star Trek iTunes app states Christopher Pike joined Starfleet in 2223, indicating that Alexander Marcus was already in Starfleet at the time and that Pike's prime counterpart was also inspired by Marcus to enlist.