Rapi: Red Hood

A Fire Code Machine Gun who takes over the Burst 1 role in lineups that leave the Burst 1 slot empty, and otherwise delivers top-tier performance as a Burst 3 main DPS.
Her Attachable Projectiles2 deal Elemental Advantage damage to Electric Code enemies, so she keeps up damage even in fights with split elements.
You have to manage Machine Gun overheating and reload timing together, so she takes more effort.

At battle start and every time Full Burst ends, her behavior branches based on the lineup. If there is no ally whose base Burst stage is 1, Combat Support, which changes her to Burst stage 1, stays active; if there is such an ally, Combat Support is removed. When Full Burst starts, in the Combat Support state all allies gain Burst Skill Cooldown reduction and increased attack damage; otherwise she gains Increased ATK and increased Damage to Interruption Parts. From battle start, against Electric Code enemies, her Converts damage to Elemental Advantage damage effect and increased attach and explosion multipliers stay active, and Attachable Projectiles2 fire every set number of normal attacks. Her Burst Skill gives herself cooldown reduction and increased explosion range, gives all allies Increased ATK based on caster's ATK, and deals additional damage to enemies near the aim point. In the same activation, increased attach damage and a relaxed trigger-count condition for Attachable Projectiles2 apply briefly all at once.

Before deploying, check your roster for an ally whose base Burst stage is 1 and choose her buff pattern accordingly.
Keep sustained fire uninterrupted to meet the Attachable Projectiles2 trigger condition.
Auto is enough by default; if overheating bothers you, just adjust your firing intervals by hand.

For keeping Machine Gun ammo up, prioritize the Bastion Cube and support Reload Speed with the Resilience Cube. For Charge Speed headroom, consider the Adjutant Cube. If the cube level gap is large, use the higher-level Harmony Cube first regardless of ranking.

Keep the Elemental Advantage damage and ATK lines, and use Max Ammo lines to stabilize her sustained-fire conditions. Split Critical Rate and Critical Damage lines according to your gear and setup.

Attachable Projectiles2 and her Burst enhancement carry a lot of weight, so raise Burst and Skill 2 first, and cover the lineup buff axis with Skill 1.

In Story, bosses, and Tower she appears in top lineups both as a Burst 1 substitute and as a Burst 3 main DPS. In Arena, results vary greatly depending on overheating and engagement range.

When the Burst 1 slot is empty she fills Burst stage 1 by herself, giving lineups more flexibility.
She deals Elemental Advantage damage to Electric Code enemies, so her Attachable Projectiles2 damage holds up even in mixed-element fights.

Machine Gun overheating can break her firing tempo.
Her Full Burst start buff pattern changes depending on whether Combat Support is active, so you have to check your roster every time.