Vesti: Tactical Upgrade

A Fire Code, Burst 3 Attacker Rocket Launcher Nikke.
Each Full Charge hit accumulates ignore-DEF damage, and her Burst enlarges explosion range to deal ignore-DEF Burst Skill damage to all enemies and parts.
She is strong against high-DEF enemies or fights with many targets, and in Main Story, normal stages, campaign, and Tower. With Emma and Eunhwa, it is easier to meet Monster Stage2's conditions as designed, but she is often used alone or in other meta lineups. She is rated poorly in single-boss fights.

Missile Guide1: on a Full Charge attack, if she is not in Missile Guide1, she gains up to three shots' worth of Missile Guide1 with Increased Charge Speed and Increased Charge Damage. Missile Guide1 is cleared when a full reload finishes.

Monster Stage2: when a Full Charge attack hits, it deals ignore-DEF damage based on final ATK to the target. If she is in Battlefield Setup at that time, she briefly gains Increased ATK, and if the target has Internal Explosive Rounds, she briefly gains Projectile Explosion Damage increase.

Missile Container OnlineB grants herself explosion range increase and ignore-DEF damage increase for ten seconds, and deals ignore-DEF Burst Skill damage based on final ATK to all enemies, including parts. Her Burst cooldown is 40 seconds.

She can be played on auto.
Full Charge hits and reload timing directly affect Missile Guide1 uptime, so timing Charge and reloads by hand is more stable to reduce buff gaps.
In the Arena, Burst Skill range greatly affects how she feels, so builds that prioritize that skill level appear.

If you aim for Monster Stage2's extra conditions (Battlefield Setup, enemy Internal Explosive Rounds), the same lineup makes it easier to meet them as designed. In practice, though, putting only Vesti into other meta lineups is more common than locking in all three.

Rocket Launcher reload speed and Max Ammo move together with Charge and reload timing, affecting stability. Prioritize reload speed and Max Ammo Harmony Cubes, and use ammo refill cubes situationally. If you have a higher-level cube, consider level before order.

Elemental Advantage damage and ATK are the baseline. Her normal attacks fire in groups of four, so builds often set Max Ammo to multiples of four. Charge Speed lines can stack with Missile Guide1 for a noticeable effect.

Monster Stage2 carries a lot of ignore-DEF damage, so raise S2 first. Her Burst combines explosion range, ignore-DEF damage increase, and enemy-wide hits. Missile Guide1 appears relatively less level-dependent, so it is sometimes put off.

Her damage is very noticeable in multi-enemy content like Main Story and normal stages, Tower, and Hard campaign with a CP gap. In boss fights, ignore-DEF matters when there are many parts, but she is rated poorly in boss fights where only a single target remains for long. In the Arena, explosion range and ignore-DEF Burst Skill damage greatly decide wins and losses.

Each Full Charge hit stacks ignore-DEF damage, so she loses less against high-DEF enemies.
After enlarging her explosion range with her Burst, she can deal ignore-DEF Burst Skill damage to all enemies.

Missile Guide1 is cleared by reloading, so Full Charge and reload patterns must line up.
Her firing pattern changes with Max Ammo settings, so gear lines must be tuned.
The explosion range and ignore-DEF damage increase from her Burst Skill are short, leaving a gap without those buffs until the next Burst.