Mica: Snow BuddyLimited

An Iron Code Burst 1 SMG Supporter who, once she builds up long streams of hits, broadly grants allies Damage Taken reduction, increased max ammo, and even an increased stack count of stackable buffs.
Her warm-up is long, so in short fights the battle can end before her stacks fill.
The Increased ATK from Snowfield FestivalB is proportional to caster's ATK, so check her own ATK gear as well.

As her sustained-fire hits build, all allies stack Damage Taken reduction, and at max stacks all allies gain a lasting Increased Max Ammunition Capacity. Every set number of hits, all allies gain an increased stack count of stackable buffs, and from battle start she gains increased Burst Gauge filling speed, so she fills Burst Gauge faster. Her Burst Skill removes one debuff from all allies and briefly grants Increased ATK proportional to caster's ATK.

Keep normal-attack hits flowing to raise Damage Taken reduction stacks to the max.
Auto fire meets the conditions by default, so auto play is enough.
Only time the Snowfield FestivalB button manually when you need debuff removal.

Reduce reload time with the Resilience Cube and cover ammo recovery with the Bastion Cube. Consider the Adjutant Cube for Charge Speed support when you have room to spare. If Harmony Cube levels differ greatly, use the higher-level cube before the ranking.

ATK lines tie directly to the Snowfield FestivalB Increased ATK multiplier. Ammo lines help meet the normal-attack hit conditions. Hit Rate lines stabilize shot grouping in fights at longer range.

S2 boosts Burst Gauge filling speed (how fast she fills Burst Gauge) and the stackable buff increase. The Damage Taken reduction and ammo increase come from S1 stacks. The Snowfield FestivalB Increased ATK multiplier follows Burst level.

Her stack warm-up is clearly valuable in Story, boss, and Tower stretches with long fights against a single enemy. When fights break off quickly, as in Arena, the battle can end before her stacks fill.

With both Damage Taken reduction and increased max ammo applied, her support range widens for ammo-focused DPS.
From battle start she gains increased Burst Gauge filling speed, so she fills Burst Gauge faster and helps the Burst 1 rotation.

She needs many normal-attack hits, so her contribution kicks in late in the fight.
The debuff removal on her Burst Skill is less useful when there are no debuffs.
She doesn't directly reduce ally Burst cooldown.