Yuni

A Burst 2 Fire Rocket Launcher Defender Nikke who grants Charge Speed every Full Burst, DEF, healing, and Max Ammo support with each Full Charge, and immobilize with her Burst.
She is rarely a main pick in Story, bosses, Raids, or the Arena.
Her buff values are low, and her real performance is weak in the latest meta's competition for healer and Burst 2 slots.

When Full Burst starts, all allies' Charge Speed rises, and each Full Charge attack briefly raises all allies' DEF, attack-damage-based healing, and Max Ammo. Her Burst Skill damages enemies in attack range and immobilizes them.

Fire after a Full Charge to cycle the SDS2 buff.
When Full Burst starts, the DMNS1 Charge Speed buff applies.
Leave her on auto by default, and time BDGB by hand only when you need immobilize.

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

For her own Full Charge and DMNS1 and SDS2 triggers, prioritize Adjutant Cube and Quantum Cube. Cover reloads with Resilience Cube. If Harmony Cube levels differ greatly, use the higher-level cube over the ranking.

Buffs and immobilize are her core, so prioritize Charge Speed lines, and put Elemental Advantage and ATK for her own damage as low priority.

If she is not a meta main, save skill books, and only when using her raise DMNS1, SDS2, and BDGB to 4. Charge Speed and Full Charge synergy are key, so raise all three skills together. Maxing to 10 offers little return for the investment.

She rarely appears in top Solo Raid lineups. Used as a substitute only when you need support for a Charge weapon squad or immobilize.

Every Full Burst, all allies' Charge Speed rises.
Each Full Charge attack raises allies' DEF, attack-damage-based healing, and Max Ammo.
Her Burst Skill can immobilize enemies in attack range.

Her buff and healing values are low, so she falls behind newer healer and Burst 2 Nikkes.
SDS2 requires allies to keep firing Full Charge shots, so it triggers inconsistently.
She is a Defender, but she cannot really tank and is rarely a main pick.