Aang, Airbending Master

When Aang enters, airbend another target creature. (Exile it. While it's exiled, its owner may cast it for {2} rather than its mana cost.)
Whenever one or more creatures you control leave the battlefield without dying, you get an experience counter.
At the beginning of your upkeep, create a 1/1 white Ally creature token for each experience counter you have.

Atreus, Impulsive Son

Reach
{3}, {T}: Draw a card for each experience counter you have, then discard a card. Atreus deals 2 damage to each opponent.
Partner—Father & son (You can have two commanders if both have this ability.)

Katara, Waterbending Master

Whenever you cast a spell during an opponent's turn, you get an experience counter.
Whenever Katara attacks, you may draw a card for each experience counter you have. If you do, discard a card.

Blade of Selves

Equipped creature has myriad. (Whenever it attacks, for each opponent other than defending player, you may create a token copy that's tapped and attacking that player or a planeswalker they control. Exile the tokens at end of combat.)
Equip {4}

Kratos, Stoic Father

Whenever you attack with one or more Gods and whenever a God dies, you get an experience counter.
At the beginning of your end step, put a number of +1/+1 counters on target creature equal to the number of experience counters you have.
Partner—Father & son (You can have two commanders if both have this ability.)

Otharri, Suns' Glory

Flying, lifelink, haste
Whenever Otharri attacks, you get an experience counter. Then create a 2/2 red Rebel creature token that's tapped and attacking for each experience counter you have.
{2}{R}{W}, Tap an untapped Rebel you control: Return this card from your graveyard to the battlefield tapped.