Build a custom person. Do not pick a canon biography and wear it like a costume, unless the table has explicitly agreed to play a named figure (not recommended for Sozin, Roku, or the next Avatar's locked-in later life).
STEP 1. Choose a people (Race). Fire National; Earth Kingdom citizen (name a region); Northern Water Tribe; Southern Water Tribe; Foggy Swamp Tribe; Air Nomad (name a temple); Sun Warrior descendant; Kyoshi Islander; Si Wong tribesperson; Lambak clan; or Colonial mixed-heritage. This sets culture, starting POI, and which rank titles you use.
STEP 2. Choose a lineage or tradition — the clan layer. Lineages are family, temple, or training traditions, not all bloodlines. Examples: Caldera noble house, Fire Sage line, Imperial Army tradition, Sun Warrior descent, Lambak spirit-metal clan, colonial house, Ba Sing Se Inner Ring family, Dai Li candidate line, Earth Sage remnant, Omashu mason-chute tradition, Kyoshi Warrior school, Si Wong sand-sail kin, Northern warrior caste, Northern healing line, Southern hunter-seafarer house, Foggy Swamp root-line, one of the four Air Temples, Guiding Wind adherent, spirit-touched, chi-blocker tradition (rare, original as a formal school), White Lotus household (secret), Yuyan trainee line. Write the lineage name on your sheet and @ its passive at session start.
STEP 3. Choose a class. Firebender, Waterbender, Earthbender, Airbender, Weapons Specialist (nonbender warrior), Chi-Blocker, or Sage. The Avatar is a special origin layered on an Air Nomad (by the cycle) plus the Avatar class, not a fifth random nation. A mixed-nation or nonbender Avatar would be an original twist and should be rare.
STEP 4. Choose one Origin Feat from the thirty-six. This is your specialization at level 1. You can take more later when 5e would grant a feat or an ability score improvement.
STEP 5. Set starting rank and equipment. Most PCs start as Initiate (levels 1-4) with the local title for their nation (Fire Recruit, Air Student, Water Pupil, Earth Student or levy). @ two to four Novice techniques. Wear nation clothes. Carry a weapon or bending focus (fans, staff, gourd, glider, sabre) and one personal keepsake.
STEP 6. Choose a starting hook and a home sub-location. Pick a nation hook and a specific stall, barracks, dormitory, dock, or kitchen. Tell the GM: I am starting at @(POI), (sub-location).
STEP 7. @ passives and decide the Avatar question as a table: no one is the Avatar; one player is the Avatar (often after a Downtime time-skip to age 8-16); or the Avatar is a named NPC infant the party will meet.
Currency is Gold piece, Silver piece, and Copper piece. Ten copper make one silver. Ten silver make one gold. A bowl of noodles is a few copper. A private inn room is about a silver. A charter boat may be two silver.