Very little is known about Michael Mercer.
He appeared on the steps of Æon's Chicago brownstone in 1945, claiming to be the son of Æon founder Maxwell Anderson Mercer. The senior Mercer was unsurprised at his appearance. The identity of Michael's mother is unknown, but it's likely that he was named after Max's friend, Michael Daemon Donighal.
When Max Mercer disappeared in 1950, Michael observed that his father would return when he was needed. It's unclear if Michael took Max's place in Æon or otherwise contributed to the Society.
Other Notes Edit
It has been suggested (by Bruce Baugh) that since Max Mercer was a jack-of-all-trades and master of disguise, Michael Mercer may, in fact, be Maxwell in disguise. In which case, Margaret is not Max's granddaughter, but his actual daughter.