RemLimited

A Water Code Machine Gun Burst 2 support Nikke.
Her Burst Skill grants her Increased Critical Rate and grants all allies wielding Rocket Launchers Increased ATK and Increased Max Ammunition Capacity, while also briefly triggering a team heal at the same time.
By design she pays off most in Rocket Launcher DPS lineups, and as a limited character she is hard to pull again, which is worth keeping in mind.

At the start of battle she gains healing proportional to Attack Damage, and continuous HP recovery is equally shared between herself and the two Rocket Launcher allies with the highest final ATK. Using her Burst Skill grants her Increased Critical Rate through Oni Aura and grants all allies wielding Rocket Launchers Increased ATK based on the caster's ATK and Increased Max Ammunition Capacity together. When she uses her Burst Skill, she also briefly applies an effect that equally shares HP recovery among all allies. While Oni Aura lasts, every set number of normal attack hits stacks Increased ATK, up to 30 stacks.

When Machine Gun overheat fills, reload to keep up continuous fire.
Her Burst Skill delivers the Rocket Launcher ally buff and a team heal together, so use it in line with the Full Burst order.
If you are going for ATK stacks after Oni Aura applies, leave her on auto, and switch to manual only when you want to time the Burst yourself.

Use reload and ammo to keep Machine Gun fire going, matching Rem's own Oni Aura and stack management. If Harmony Cube levels differ a lot, use the higher-level cube ahead of the theoretical ranking.

Set Elemental Advantage damage and ATK as the baseline. As a Machine Gun she takes ammo, and adds Critical options to pair with Oni Aura.

Her Burst Skill is the core of Oni Aura and the Rocket Launcher ally buff, so prioritize the Burst, then raise the active skill that holds continuous healing and ATK stacking.

Placing Rocket Launcher DPS in the same squad makes her buffs and continuous healing land on the intended targets. She is a pure supporter in stats, so main firepower usually comes from Rocket Launcher allies.

She grants all allies wielding Rocket Launchers Increased ATK and Increased Max Ammunition Capacity.
She heals allies through multiple routes with her Burst Skill and lasting effects.
During Oni Aura, normal-attack hits build Increased ATK stacks.

You have to alternate between Machine Gun overheating and reloading.
Her buffs and continuous healing need to match a Rocket Launcher lineup, so the payoff drops with different weapons.

A limited collab character.