iDoll Ocean

A Burst 1 Water Submachine Gun healer, but her healing scales with the caster's final max HP, so the numbers are low.
She has no core use in story, bosses, or Raids.
With R rarity, she has almost no investment value next to top healers like Rapunzel and Marciana.

When the last bullet hits, she heals the single ally with the lowest remaining HP ratio based on the caster's final max HP. S2 heals the lowest-HP ally again in the same way, and her Burst applies the same ratio healing to all allies.

Keep up sustained fire and empty the magazine before reloading so the last bullet hits.
Use S2 when an ally has low HP, and use Burst when the whole squad needs healing.
Keep her on auto, and adjust reload and ammo only if the last bullet often misses.

There is little evidence in the recent meta for pairings that run her in the same squad.

To land last-bullet heals often, adjust reload with Resilience Cube. Her healing scales with the caster's HP, so Vigor Cube and Healing Cube are secondary. If Harmony Cube levels differ a lot, use the higher-level cube over the ranking.

As an R healer, Overload investment has low value. If you use her, secure caster survival with DEF, and as a Submachine Gun, add a few Max Ammo and Hit Rate lines to help land the last bullet.

She is not a meta core, so she barely uses skill books. If you must, raise Ocean DiveB, Ocean Cleansing2, then Ocean Shower1.

She does not appear in top Solo Raid teams. Use her only as a temporary healer early on when your roster is thin.

Landing the last bullet heals the ally with the lowest HP.
S2 heals the same target again.
Her Burst heals all allies.

Her healing scales with the caster's final max HP, so it is small on an R stat profile.
The last-bullet condition means she must spend ammo.
An R Nikke with no core use compared to top healers.