You may choose any of the two..
Requirements for Kamui:
Age: Atleast 25-27 years old
Abilities: It's up to you if you want him to be an Earth or Water Bender but he must be skilled in blacksmithing.
Appearance: (Youth about 18-20 years old) Short brown hair, blue eyes, long face, skinny but with a slightly muscular arm like Sokka. (Present age) Long brown hair, tall and slightly muscular.
History: Kamui is an orphaned blacksmith in the Water Tribe. He has a strong love for Kendara Sai, a daughter of a very wealthy family. Kendara's parents never wanted him to be their son-in-law so they did everything to stop the two lovers. One night, after he and Kendara planned to run away, Kendara's parents kidnapped him, beat him almost to death and threw his body in the ocean. a fisherman from the Earth Kingdom found him while fishing near where Kamui's body is thrown. The fisherman adopted him. He couldn't stop thinking about Kendara and wanted to come back to the Southern Water Tribe to get her but the old fisherman stopped him for he was still young that time. After 4 years, the fisherman died, leaving all his belongings to Kamui. (Now, it's up to you if you want Kamui to be rich or what. Make his next story)
-Kendara will be also found in the same area where Kamui is residing. The two will meet someday or another.
Requirement for Kamui's look-alike:
Name: (You name him, it's your choice)
Age: must be 25-27, too
Appearance: he must have short hair, blue eyes, long face, and he must be tall. The rest will be up to you.
Residence: Earth Kingdom
Abilities: Anything. But he must be good at blacksmithing, too
Storyline: In the middle of Kendara's craziness, she will meet this guy and thinks him as Kamui. She will be following him (Of course, the guy will be annoyed). But after sometime, he will listen to her and will do anything to cure her addiction. (It's up you if you want to be with Kendara forever or just help her back to her senses.)