Mary

In the current meta she is hard to use as a main Burst 1 healer.
CPR1, Nursing2, and Angel in WhiteB provide healing, increased HP received, and DEF, but her firepower and buff range are narrow.
She is outclassed by top healers and buffers like Pepper, Liter, and Mary: Bay Goddess.

When the last bullet hits, CPR1 heals the 1 ally with the lowest remaining HP ratio proportional to the caster's max HP. Every time Full Burst starts, Nursing2 raises received HP recovery for all allies for 15 seconds.

Angel in WhiteB heals all allies and, if her own HP is 50% or higher, applies a DEF buff. She is a Burst 1 Shotgun support with a 40-second cooldown.

Adjust aim and range so the Shotgun's last bullet hits the enemy.
Nursing2 and Angel in WhiteB are fine on auto.
To make CPR1 land on a low-HP ally, time only the last bullet manually.

There is no fixed pairing in recent top Solo Raid or boss lineups. Only slot her to fill the Burst 1 healer slot on early accounts.

For healing output use the Healing Cube and Vigor Cube, and for the last-bullet CPR1 cycle use the Resilience Cube to shorten reload. If you have high-level Harmony Cubes, look at level before the order.

Skip DPS lines like Elemental Advantage damage and ATK. Use max HP, DEF, and ammo to raise CPR1 and Angel in WhiteB healing and last-bullet frequency.

If she is not a meta core pick, leave her at 1/1/1. Only when you use her, bringing CPR1, Nursing2, and Angel in WhiteB to 4 is enough.

She almost never makes main Solo Raid, boss, or Arena lineups. She is closer to a healer substitute for early Campaign and account growth.

Last-bullet CPR1 heals the 1 ally with the lowest HP.
Nursing2 raises received HP recovery every Full Burst.
Angel in WhiteB heals all allies and grants a DEF buff.

Her CPR1 depends on the Shotgun last-bullet condition, so it often misses.
Her Burst cooldown is 40 seconds and her personal firepower is low.
Compared with Pepper, Liter, Mary: Bay Goddess, and others, her healing and buff value is low.

Mary: Bay Goddess is a separate Nikke; for long boss fights and Raid support, see her guide instead.