Aria

A Water Burst 2 Machine Gun support Nikke who applies ally crit buffs every Full Burst and a shield with her Burst.
She is rarely a core pick for story, bosses, Raids, or Arena.
She is rated low in both recent PvE and PvP, so unless she is a favorite, her investment priority is low.

When Full Burst starts, all allies get Increased Critical Damage, and each time the last bullet hits, Increased Critical Rate follows briefly. Using her Burst Skill gives all allies a shield proportional to her final max HP, and gives herself Increased Hit Rate. The more often her Machine Gun lands last bullets, the longer the squad's crit buffs continue.

Cycle reloads and ammo so the last bullet hits without breaking sustained fire.
When Full Burst starts, the Allegro1 buff applies.
Keep her on auto and only time reloads by hand before the magazine empties.

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

To empty the magazine quickly and land last bullets often, Resilience Cube for reload comes first. The shield scales with final max HP, so Vigor Cube and Bastion Cube are secondary. If Harmony Cube levels differ a lot, use the higher-level cube over the ranking.

Her personal damage contribution is small. To raise shield value, prioritize DEF and HP lines, and add Elemental Advantage damage and ATK later.

If she is not a meta core, save skill books and raise Allegro1, Lacrimoso2, and Burst to 4 only when using her. Maxing to 10 gives little return for the investment.

In past Special Interception there was a double-shield team that withstood Modernia's laser, but it is rarely used now. Her share is very low even in top Solo Raid teams.

Every Full Burst, all allies get Increased Critical Damage.
When the last bullet hits, allies' Critical Rate rises.
Her Burst can shield all allies.

Burst 2 slot competition is fierce relative to her buff values, so she is rarely a core pick.
Her personal damage and Hit Rate buff alone fall behind meta supporters and tanks.
As a Machine Gun, she needs reload and ammo management to reliably meet last-bullet conditions.