Little Mermaid

A top-tier Wind Code Burst 1 support. As a single Burst 1 she shortens allies' Burst Skill Cooldown, fills the Burst Gauge, and raises allies' attack damage during Full Burst.
However, Bubble Order1's Focuses fire only applies to all allies while she is Focusing, so the key is keeping Focusing uninterrupted. If Focusing stays stable on Auto alone, there's no need to hold her manually to the end; if you want to pick which enemy to hit, aim yourself at that point.
When elements line up, her support effects and Bubble use clearly come alive in Raids and boss fights against Iron Code enemies (Wind advantage). Placing ammo-hungry allies like MGs in the lineup also triggers her Bubble effects a bit faster.

Bubble Order1 grants Focuses fire to all allies only while she is Focusing. When Full Burst Time starts, it gives all allies increased attack damage, and when Full Burst Time ends, it reduces all allies' Burst Skill Cooldown. Each time the total ammo consumed by allies reaches 400 rounds, it fills all allies' Burst Gauge by a set ratio.

Bubble Wave2 applies increased damage taken as a Bubble status when enemies appear. When normal attacks hit a set number of times and the target still has Bubble, it changes to Popped Bubble, applying further increased damage taken and a brief stun, then removes the Bubble. During Full Burst Time only, it Attacks sequentially on random enemies several times per second, and each time the total ammo consumed by allies reaches 500 rounds, it adds a Bubble barrage of sequential attacks on random enemies.

Siren's SongB gives all allies increased attack damage and bullet recovery, and at the same time applies Increased ATK based on caster's ATK to herself.

To use Focuses fire reliably, just keep Focusing on Little Mermaid. If Focusing holds well on Auto, there's no need to hold her manually to the end.
During Full Burst, put ammo-hungry allies like MGs in the same lineup to meet Bubble Order1's 400-round condition and Bubble Wave2's 500-round condition.
If Auto switches Focusing often, Focuses fire breaks. If you plan to hit a specific enemy, take control of Little Mermaid at that point and aim yourself.

The Bastion Cube refills ammo during sustained fire to keep her SMG shots landing. Use the Resilience Cube to reduce reload frequency, and if that's not enough, supplement with an increased Max Ammo Harmony Cube. If the cube level gap is large, use the higher-level cube first regardless of ranking.

Set Elemental Advantage damage and ATK as a baseline. Given her Submachine Gun pellet traits, it's better to also secure Hit Rate lines, and tune the rest of her weapon stats with crit and Max Ammo.

Bubble Order1 carries both Burst Cooldown and Burst Gauge support, so max Skill 1 first. Bubble Wave2 handles the debuff and most of her output from Attacks sequentially, so raise Skill 2 next. Siren's SongB's bullet recovery and ATK support get filled in last with leftover resources.

Frequently a top Burst 1 Supporter pick in PvE Raids, bosses, Story, and Tower. Because she applies Bubble when enemies appear, she is also often tried out in attack-focused Arena lineups.

Burst Cooldown reduction and an effect that Fills Burst Gauge speed up her rotations.
With Bubble and increased attack damage on top, she covers both damage taken and ally buffs at once.

Bubble Wave2's random-target Attacks sequentially effect tends to spread some shots away from a fixed single boss.
If Auto switches Focusing often, the Focuses fire effect breaks and Bubble Order1's advantage shrinks.