Mercer was a Human doctor who worked for the xenophobic terrorist group Terra Prime, based out of the Orpheus Mining Colony on the moon. In that capacity, he was responsible for the creation of a binary clone from the genetic material of Commanders Charles Tucker and T'Pol.
John Frederick Paxton, Terra Prime's leader, intended to use the baby to incite xenophobic reactions amongst Humans and then destroy her. Paxton questioned Mercer's loyalty, and began to suspect that Mercer was becoming too attached to the baby. Believing that Mercer may be a liability, Paxton had Mercer killed, his death made to look like a mining accident. (ENT: "Demons")