Yan

A Burst 1 Fire Rocket Launcher support Nikke who buffs allies' Charge Damage every Full Burst and grants ATK and Crit buffs with each of her own Full Charges.
She is rarely a main pick in Story, bosses, Raids, or the Arena.
Competition for the Burst 1 slot is fierce relative to her buff values, so she is low investment priority.

When Full Burst starts, all allies gain Increased Charge Damage. Each time she attacks with Full Charge, all allies briefly gain ATK and Increased Critical Rate. Her Burst Skill damages enemies in attack range and briefly forces them toward the center of the attack range.

Fire after a Full Charge to cycle The More, the Merrier2's buff.
When Full Burst starts, the Struck Dumb1 buff applies.
Leave her on auto by default, and time her Burst by hand only when crowd control is needed.

There is not enough evidence in the recent meta for pairing her in the same squad.

The More, the Merrier2 needs her own Full Charge, so tune Charge and Burst Gauge with Adjutant Cube and Quantum Cube. Cover reloads with Resilience Cube. If Harmony Cube levels differ greatly, use the higher-level cube over the ranking.

Her own damage contribution is small. To raise Full Charge frequency, prioritize Charge Speed lines, and add Elemental Advantage damage and ATK later.

If she is not a meta main, save skill books, and only when using her raise Struck Dumb1, The More, the Merrier2, and Fat CatB to 4. Maxing to 10 offers little return for the investment.

She rarely appears in top Solo Raid lineups. Burst 1 CDR and buffer Nikkes take that slot more often.

Every Full Burst, all allies gain Increased Charge Damage.
Each Full Charge attack raises allies' ATK and Crit Rate.
Her Burst can briefly pull enemies in range toward the center.

Her buff values are low, so she loses out in the Burst 1 slot competition.
The More, the Merrier2 requires her own Full Charge, so the buff drops easily.
Her own damage and crowd control contributions are small, so she is rarely a main pick.