Creatures you control have "Whenever this creature becomes the target of a spell or ability for the first time each turn, counter that spell or ability."
Kira protects your board by giving every creature a one-shot ward against targeted spells and abilities, then you grind out the game with evasive threats, card advantage, and counterspells while opponents waste removal. You play a tempo-control shell, deploy efficient creatures that opponents can't easily interact with, and close with unblockable beaters or a protected wincon.
Hard to remove your creatures one-for-one, since the first target each turn gets countered
Shuts down targeted edicts, sacrifice 'pings,' Aura/equipment-stealing effects, and 'destroy target creature' removal
Mono-blue gives access to the best counterspells, card draw, and evasion in the format
Excellent protective shell for fragile combo pieces, Voltron carriers, or value creatures
Does nothing against board wipes, edicts that don't target, or -X/-X effects
Commander itself is a 3/2 flier with no inherent advantage engine, so the deck can stall without support
Mono-blue lacks efficient creature removal and struggles to break parity once behind
Opponents can simply 'waste' a cheap spell to clear the ward, then resolve their real removal
Vulnerable to artifact/enchantment-based removal and damage-based wraths
Floods the board with protected spirit tokens that snowball under Kira.
Lean into a clear wincon—either a protected Thassa's Oracle/Laboratory Maniac combo or a flying-aggro Voltron line—rather than playing generic goodstuff. Add fast mana (Sol Ring, Mana Crystals, Mana Vault) and the full suite of efficient counterspells (Fierce Guardianship, Swan Song, Mana Drain) to leverage Kira's protection into a true control plan. Prioritize creatures that generate value or evasion so the board does more than just survive.