Rei

A Water Code Burst 1 Defender whose only job is covering allies with Decoy, taunt, and Damage Taken reduction.
She is rarely a core pick in the current meta, and if you have no plans to use her, you can keep her development priority low.

At the start of battle, Senior Spirit2 places a Decoy proportional to her max HP, and after 60 normal attacks, Senior Support1 fills the Burst Gauge for all allies and heals the Decoy. Her Burst, Model Senior StudentB, grants her taunt and Damage Taken reduction, and if a Decoy is up, restores decoy HP at short intervals.

Decoy HP and healing are tied to Rei's max HP. Her Burst cooldown is 20 seconds.

Keep firing to meet the 60-normal-attack condition.
Before allies take hits, use her Burst to apply taunt and damage reduction, alternating with the Decoy.
Auto is fine by default.

There are almost no pairings that see use together in recent Raid or Solo Raid main lineups. She is only occasionally slotted for survival at CP walls.

Decoy HP is tied to max HP, so prioritize the Vigor Cube. Cover Damage Taken reduction with the Tempering Cube and DEF with the Endurance Cube. If Harmony Cube levels differ a lot, use the higher-level cube ahead of the ranking.

As a Defender, prioritize Increased DEF lines. Use leftover lines on ammo to keep up fire and meet the 60-shot condition faster. ATK and Elemental Advantage damage matter less for tanking.

If you do not plan to use her, leave her at 1/1/1. When leveling, first raise S2, which boosts Decoy HP, and the Burst's taunt and damage reduction. S1, which fills Burst Gauge, is a secondary effect.

She rarely makes main Campaign, boss, Tribe Tower, or Arena lineups. She is only occasionally used for Decoy tanking in sections where your CP falls short.

From the start of battle, her Decoy absorbs damage.
Every 60 shots, she fills the Burst Gauge for all allies.
Her Burst applies taunt, Damage Taken reduction, and restores decoy HP all at once.

Her firepower and support contribution are low for current content.
The 60-shot condition means her Burst Gauge support comes in late.
Compared with newer Burst 1 units and tanks, her survival and lineup value fall short.

She was once used for Decoy HP tanking at past CP walls, but there are now many replacement Nikkes.