Emma

A Burst 1 Fire Machine Gun healer Nikke who provides HP-proportional healing and a received-healing buff.
She's rarely a main pick in Story, bosses, or Raids.
Her on-hit healing rarely triggers in PvE, so her healing contribution is unreliable.

When her HP is at 90% or higher, all allies' received healing increases. When hit, she has a low chance to heal all allies based on the caster's max HP, and using her Burst Skill grants all allies instant healing plus healing proportional to attack damage for 5 seconds.

Keep her HP at 90% or higher so the Temptation2 buff doesn't drop.
Use her Burst Skill, AltruismB, when allies' HP is low.
Leave her on auto by default; in PvE she takes few hits, so Cheerleading1 healing barely triggers.

There's little evidence in the recent meta for pairing her in the same squad.

The Healing Cube boosts the healing she gives, and the Vigor Cube boosts the final max HP her healing scales on. The Tempering Cube reduces damage taken so she lasts until healing is needed. If Harmony Cube levels differ greatly, use the higher-level cube over the ranking.

Healing scales with final max HP, so prioritize DEF lines and put Elemental Advantage and ATK for her own shooting lower.

If she isn't a meta main, save your skill manuals and only raise AltruismB, Temptation2, and Cheerleading1 to 4 when you use her. Burst and Temptation2 are her healing core, while Cheerleading1 depends on taking hits, so it comes last. Maxing to 10 gives little return for the investment.

She's almost never seen in top Solo Raid lineups. In early Arena, she's occasionally used for her sustained healing.

Her Burst Skill can give all allies instant healing plus healing proportional to attack damage.
When her HP is high, allies' received healing goes up.
When hit, she can heal all allies.

Cheerleading1 requires taking hits, so it rarely triggers in PvE.
Temptation2 only holds while her HP is at 90% or higher.
Among Burst 1 healer and CDR Nikkes, she's rarely picked as a main.