E is a member of the Endwalker cult.
Background[]
E, if that is his real name, was brainwashed or willingly joined the Endwalkers before Elysium was founded.
The Ceremony[]
E sent a small group of Endwalkers to capture a human for their invitation ceremony to be sacrificed. But the group was captured by Baron Randall Huntly and his men. This allowed the Royal Army of Elysium to locate their base under the second tallest tree. David disguised himself along with two others and successfully infiltrated the cult's ceremony.
Fate[]
After Sakamoto knocked out David with a spell, the Endwalkers battled the Royal Army of Elysium at End River. It is unknown what became of E or if he even survived the battle.