Ethan Carter Wate is Ethan's great-great-great-great-uncle (seriously, he's great) who pops up during the Civil War flashbacks.
We only see Ethan Carter Wate for a few minutes before he dies, is resurrected, then dies again. (But we do witness that, which is pretty neat.)
Here's what we know about this guy:
Here's Uncle Ethan's take on the conflict: "Most a the boys fightin' with me didn't even realize what this war is about—that they're just spillin' their blood over cotton" (9.12.176). Way to boil down four years of bloody combat and over 600,000 deaths into eight words, ECW.
Our narrator Ethan was named after his great-great-great-great-uncle, probably because his parents admired ECW's resolve. No matter what his rationale was, this nineteenth-century man deserted instead of fighting a war he didn't believe in.
Besides their names, how are Ethan and ECW similar? Let's take a look. ECW knew about Genevieve's magical background. In a letter, he wrote, "Though merely Mortal, my heart breaks with such pain at the thought of spending the rest of my life without you, Genevieve" (10.13.136). By the way he capitalized Mortal, we can infer that he knew she was a Caster.
All of this sounds suspiciously like our narrator Ethan, just a little more dramatic. The knowledge of their magical ladies didn't stop ECW or Ethan from pursuing the women they loved.
P.S. ECW was the one who gave Genevieve the cursed locket that transports Ethan and Lena into the past. Although we haven't seen his ghost yet, we wouldn't be surprised if it's lurking nearby…