Duophysite must mean "info dump" in Meronym's language, because that's pretty much all he does.
When Zachry rifles through Meronym's stuff and finds her orison, here comes Duophysite to ruin all the fun (and to let us know what the heck an orison is). Later, after Meronym has rescued Zachry from the slave runners, Duophysite pops up to burst our heroes' bubbles yet again. The Prescient ship is gone and pretty much all the Prescients, including Meronym's son, are presumed dead. Way to be a buzzkill, dude.
Anyway, Duophysite gives Zachry's narrative some direction in its final act. When Zachry finds that all his family is dead, too, it leaves him no other choice but to go with Meronym to Maui.
The word "duophysite" refers to someone who believes that Jesus Christ has two natures—one divine, and one human. Do you think this has anything to do with Duophysite, the character, or does the name Duophysite just sound cool?