Delta: Ninja Thief

Delta: Ninja Thief is a 40-second Burst II Defender, so rather than a Burst pillar she is closer to a support card that uses the Damage Taken increase debuff triggered on Full Burst entry and her squad-dependent tanking/healing mechanics.
On early-to-mid accounts she is a usable filler substitute, but on accounts that already have top general-purpose B2 buffers she is not a high priority.

The key is Ninjutsu Camouflage1's squad branch.
If there is no other Defender, she stands in as a tank with Taunt (taunts all enemies) and shield refresh; if another Defender is present, Ninjutsu Camouflage1/Ninjutsu Injection shift her role toward hit-evasion conditions and periodic healing.
Ninjutsu Acid Bomb is her core off-burst value, applying a Damage Taken increase to all enemies on Full Burst entry, while her Burst, Secret Technique: Ninja Overdrive2, has a 40-second cooldown and usually sees only supporting use.

Auto play works by default.
The key is not to run Delta as a lone Burst II, but to pair her with a 20-second Burst II to keep the cycle going.
If manual play is needed, only adjust placement/Burst timing on stages that need taunt tanking.

MG handling stability and survival come first; prioritize tanking and sustain contribution over maximizing her own damage.

For filler or support use, overinvesting is inefficient, so a basic setup is fine; if you actually use her for tanking, check the survival breakpoints before Hit Rate/ammo.

Ninjutsu Acid Bomb (S1) is the top priority. Invest more in S2 at tanking/healing breakpoints; her Burst is inefficient, so heavy investment there is low priority.

She has a place in Campaign, bosses, and Tribe Tower as a lone Defender taunt tank or an off-burst Damage Taken increase support. However, her priority in PvP and the general meta is fairly low.

She reliably provides a Damage Taken increase debuff linked to Full Burst entry, even off-burst.
Depending on the squad, she can serve as a taunt tank or a healing support, giving her some role flexibility.
She relies little on her Burst, so she works in a filler slot.

Her Burst II runs on a 40-second cooldown, making a solo B2 setup difficult.
Her Burst itself is not very efficient, so she drops in priority compared to top B2 buffers.
Her healing scales with ATK, so it feels weaker in high-deficit content.