Rouge

She's fairly widely usable as Burst 1 support in Story, bosses, and Raids.
She performs better when she receives healing and debuff support in the Rouge-Blanc (Rouge + Blanc) pairing, but her coin warm-up is slow, so she feels weak early.
In Arena, the same role has almost no value.

In the back row, Coin Flip2 cycles and grants herself and the allies on either side Sword Coin (increased attack damage). After a set number of Full Charge attacks, the same targets gain Shield Coin (Damage Taken reduction), and after a set number of Burst Skill uses, all allies gain Double Sword Coin (max HP increase based on the caster). Card Throw1, every set number of Full Charge attacks, briefly grants all allies a max HP increase based on the caster and Burst Skill cooldown reduction. Game MasterB grants all allies Increased ATK based on caster's ATK and layers on additional caster max-HP effects depending on which coins are active.

Place her in a back-row slot, with the allies who should receive buffs on either side of her.
Full Charge shots count toward Card Throw1 even if they miss. To tighten the Burst cooldown reduction cycle, keep aiming and land Full Charges quickly.
Sniper Rifle Full Charge intervals are inconsistent on auto alone, so for a stable cycle, time the Full Charge release manually.

Rapid Full Charges and reload tie directly to the Card Throw1 cycle. Go in order: Resilience Cube for reload speed, Wingman Cube for max ammo, and Bastion Cube for ammo refill. If Harmony Cube levels differ greatly, use the higher-level cube before the ranking.

Card Throw1 is based on Full Charge hit count, so having both ammo and Charge Speed lines helps keep the cycle on track.

Put levels into Card Throw1 first to secure its Burst Skill cooldown reduction values. Coin Flip2's attack damage increase, damage reduction, and max HP increase last a long time, so raise it next. Game MasterB is the core of her Increased ATK and max HP buffs, so it's fine to fill it last.

She's discussed on the assumption that she provides Burst cooldown reduction plus buffs and survival support in Story, bosses, Union Raid, and similar content. She can be used in Tower on the same support assumption. In Arena, Burst cooldown reduction is worth little and her coins build slowly, so her priority is low.

Full Charges alone can provide Burst cooldown reduction for all allies.
If you just place her in the back row, Sword Coin and Double Sword Coin effects deliver attack, defense, and HP support in sequence.
In the Rouge-Blanc pairing, her timing lines up well with Blanc's passive Burst cooldown reduction.

Before reaching the Shield Coin stage, it's hard to benefit from her Burst healing effect.
Card Throw1 and Coin Flip2 only turn on their main effects after enough Full Charges and Bursts, so they feel weak early in battle.
In short fights like Arena, almost none of the coin buildup or Burst cooldown reduction benefit remains.