Whenever Haliya enters or attacks, put a +1/+1 counter on target creature you control.
Whenever one or more creatures you control with +1/+1 counters on them deal combat damage to a player, draw a card.
Build a wide-and-tall board of creatures wearing +1/+1 counters, then turn every combat into card advantage by connecting with at least one counter-bearing attacker. Haliya seeds counters each time she enters or attacks, and you snowball with proliferate, doublers, and anthem effects until you simply overrun the table.
Built-in card advantage that refuels you every combat, fighting the natural attrition of going wide
Lives in GW, the premier color pair for +1/+1 counter synergies, ramp, and protection
Counter placement targets any creature you control, enabling Voltron or go-wide pivots
Recovers well from board wipes since recasting her immediately drops a counter again
The draw trigger requires connecting in combat, so blockers, fog, and pillowfort effects can shut off your engine
Highly reliant on the commander herself for the payoff—repeated removal taxes you heavily
No built-in evasion; a single big blocker can stonewall your draw trigger
Five mana is slow, and the deck wants her down early to start snowballing