Santa Ellia is a Christmas Limited Edition decoration monster from "Light up the Christmas Tree" event held on Dec 18th - 31th 2014. She can be obtained as a reward when a player has completed all the missions.
Another event, "It's okay even if it doesn't snow!", from which she can be obtained was held from Dec 21st, 2015 - Jan 3rd, 2016. She will appear in the player's inbox after the snowman's mouth is completed.
Santa Ellia will increase your MAX energy by 1 and will roam around the village with your other monsters. (Please note that Ellia can't participate in battles.) She will be near the Summoner's Tower when first obtained.