Joan — Alter

Here is a helpful summary of her character and usage in FGO:

A younger, comical "Santa" version exists, known as Santa Alter Lily. Joan Alter

As an Avenger, she is a high-damage dealer, often noted for having some of the highest Attack (ATK) values in the game. Key Skills (Leveling Priority): Here is a helpful summary of her character

A party-wide damage boost that doubles for [Dragon] trait allies (level last). comical "Santa" version exists

She is a heavily featured character in fan art, merchandise, and cosplays.