Egro is brothers with Papyrus and Sans. He used to be named Roman, and was a skeleton. Sans and Papyrus were a bit rude to them, and Egro got angry. They used to study humans, that's why they were a bit rude to Egro. Later on, they finally got tired of it. Egro locked themselves in their own room, but then was kicked out of the house. Having no other place to live, they travelled. Egro went to Alphys' lab, and asked her to change themselves into a god. Alphys refused, but Egro managed to steal the 8 souls and become a god. They then set out to kill Papyrus and Sans. If you gave your soul to Omega Flowey, they will say "Did he take your soul, too? I knew he was evil." and their fight is skipped.
Egro's fight is the hardest in the game, switching to a platformer at some parts.