Hada Freeblade

Whenever this creature or another Ally you control enters, you may put a +1/+1 counter on this creature.

Katara, Water Tribe's Hope

Vigilance
When Katara enters, create a 1/1 white Ally creature token.
Waterbend {X}: Creatures you control have base power and toughness X/X until end of turn. X can't be 0. Activate only during your turn. (While paying a waterbend cost, you can tap your artifacts and creatures to help. Each one pays for {1}.)

Hero of Goma Fada

Rally — Whenever this creature or another Ally you control enters, creatures you control gain indestructible until end of turn.

Lantern Scout

Rally — Whenever this creature or another Ally you control enters, creatures you control gain lifelink until end of turn.

Hook Swords

When this Equipment enters, create a 1/1 white Ally creature token, then attach this Equipment to it.
During your turn, equipped creature gets +1/+1 and has first strike.
Equip {3}

Kazandu Blademaster

First strike, vigilance
Whenever this creature or another Ally you control enters, you may put a +1/+1 counter on this creature.

Kyoshi Battle Fan

When this Equipment enters, create a 1/1 white Ally creature token, then attach this Equipment to it.
Equipped creature gets +1/+0.
Equip {2} ({2}:Attach to target creature you control. Equip only as a sorcery.)

Appa, the Vigilant

Flying, vigilance
Whenever Appa or another Ally you control enters, creatures you control get +1/+1 and gain flying and vigilance until end of turn.

Allied Teamwork

When this enchantment enters, create a 1/1 white Ally creature token.
Allies you control get +1/+1.

Avatar Enthusiasts

Whenever another Ally you control enters, put a +1/+1 counter on this creature.

Earth Kingdom Protectors

Vigilance
Sacrifice this creature: Another target Ally you control gains indestructible until end of turn. (Damage and effects that say "destroy" don't destroy it.)

Munda's Vanguard

Cohort — {T}, Tap an untapped Ally you control: Put a +1/+1 counter on each creature you control.

Angelic Captain

Flying
Whenever this creature attacks, it gets +1/+1 until end of turn for each other attacking Ally.

Sokka, Tenacious Tactician

Menace, prowess (Whenever you cast a noncreature spell, this creature gets +1/+1 until end of turn.)
Other Allies you control have menace and prowess.
Whenever you cast a noncreature spell, create a 1/1 white Ally creature token.

Akoum Battlesinger

Haste
Whenever this creature or another Ally you control enters, you may have Ally creatures you control get +1/+0 until end of turn.