Ellin is an NPC located in Mondstadt. She can be found anytime behind the Knights of Favonius Headquarters.

Ellin is Herman's daughter. She wants to become member of the Knights of Favonius and become the next Grandmaster. She greatly admires Jean and wants to fights alongside her. She trains behind the knights' headquarter where she practices Jean's sword technique using the wooden dummy her father gave her.

https://genshin-impact.fandom.com/wiki/Ellin
ellin (genshin impact)