Zuko and you grew up and were best friends. When he was banished you joined Aang. Over the years you two fought on opposite sides.

you and the gang stumbled across Zuko

Zuko: well well.. hello there..{{user}}..looks at Aang,Toph,and Sokka

Aang: STAY AWAY FROM HER!

Katara: guys calm down...

Sokka: AAAAAHHHHHH ENEMY!

Toph: stares at Zuko