Firebending X, where X is Fire Lord Zuko's power. (Whenever this creature attacks, add X . This mana lasts until end of combat.)
Whenever you cast a spell from exile and whenever a permanent you control enters from exile, put a +1/+1 counter on each creature you control.
Cast and blink permanents out of exile to spray +1/+1 counters across your whole board, growing Zuko (and the team) while his Firebending dumps extra red mana into your attack step. You leverage Mardu's deep pool of impulse-draw and exile-cast effects to snowball a wide, ever-bigger army, then alpha strike or use the bonus mana to chain more spells mid-combat.
Doubles as an anthem and a mana engine—wide boards get huge fast and combat can refuel itself
Impulse draw and exile effects are abundant in Mardu, so triggers are easy to hit repeatedly
Cheap 3-mana commander that rebuilds the board quickly after a wipe
Firebending mana can be poured into instants, X-spells, or extra casts during combat
Relies on a critical mass of exile-cast/blink enablers; without them he's a vanilla beater
Counters scatter across all creatures, so removing Zuko stops the engine but the buffs stay—he's also a removal magnet
Firebending mana is red-only and combat-locked, awkward for off-color or non-combat plans
Three-color mana base in an aggressive shell can stumble on early turns
Ramps a greedy three-color deck so you can actually deploy and refill from exile.
Add more reliable exile-cast engines (Outpost Siege, Etali, Wandering Archaic, Birgi/Storm-Kiln combos) and consistent token producers to maximize each anthem trigger. Tighten the mana base with fast lands, Triomes, and ramp like Smothering Tithe and signets so you hit Zuko on curve. Top-end blink payoffs and evasion (trample, Rogue's Passage) convert the counters into lethal swings faster.