When Orpheus first appeared in 1986, he had been Roman's former ISA partner. His wife was Rebecca, who was accidentally killed by Roman while on a mission.
Orpheus came to Salem to get revenge on Roman by kidnapping Marlena (and letting everyone think she was dead), taking her to an island and making her become a new "mother" to his son and daughter. I guess we're supposed to believe Evan/Christian is that same boy (whose age has apparently been frozen in time, since he should be around the same age as Sami and Eric), unless he was a child born to Orpheus after those two who were on the island (and were never given names on the show).