Ludmilla

She is not a meta core Burst 1 defender. Her key trait is Queenly Disposition2's all-enemy taunt and Damage Taken reduction for herself at the start of Full Burst.
Noblesse Oblige1 lowers the target's DEF and ATK when her last bullet hits. She has no self-heal, so she needs a lineup with healing and shields.

Her structure: sustained fire puts Noblesse Oblige1 on her last bullet, Queenly Disposition2 activates an all-enemy taunt and a survival buff for herself when Full Burst Time starts, and then Snow QueenB reinforces with AoE damage and ally DEF.

Snow QueenB deals damage to the top 10 enemies by final ATK, and if her HP is 50% or higher, all allies' DEF rises. Her Burst cooldown is 20 sec.

Aim right before reloading so her last bullet hits the target.
Using Snow QueenB right before or after Full Burst Time overlaps the taunt and DEF buff timing.
Sustained fire can be left on Auto, but her last bullet and Burst are better handled manually.

There are almost no combos fixed in recent core Raid, meta boss, or Arena squads. She is usually paired with healing and shield Nikkes.

Reload timing matters for the last-bullet debuff, so go Resilience Cube, Vigor Cube, then Tempering Cube. If you have a high-level Bastion Cube, use it as the second option.

As a defender, prioritize 4 lines of Increased DEF. Some add 1-2 Max Ammo lines to control the last bullet. Skip Elemental Advantage and ATK end-game lines.

If she isn't a meta core pick, leave her at 1/1/1. When raising her, level Queenly Disposition2, Noblesse Oblige1, and Snow QueenB evenly to 4.

She is not a core Burst 1 for Story, bosses, Tower, or Arena. She is for support or preference lineups that need taunt and debuffs. She is almost never used in Solo Raid.

At the start of Full Burst, Queenly Disposition2 applies taunt to all enemies and Damage Taken reduction to herself.
When her last bullet hits, Noblesse Oblige1 lowers the target's DEF and ATK.
Snow QueenB deals AoE damage and gives allies a DEF buff when her HP is 50% or higher.

She has no self-heal or shield, so without healing or protection in the lineup she struggles to last.
You have to time her last bullet and Full Burst, so she takes effort.
She's rarely a core pick for Raids, meta bosses, or Arena.

Her skills and role differ from Ludmilla: Winter Owner. See the separate guide for Winter Owner.