Sin: Swift Bunny

A Water Burst 3 DPS who uses the same Bunny Mode as Guilty: Mighty Bunny; in Solo Raid against Fire enemies (= Water advantage), she appeared in most top records alongside Guilty in a DEF-ignoring squad.
Her peak firepower is a bit lower than Guilty's, but her all-enemy Burst lets her work in stages and the Missilis Tower, and she is easier to handle.
Her firepower is not high enough to replace top Water DPS units, so unless your account aims for rankings in Water-weakness Solo Raids, her priority is low.

Like Guilty, she starts in Bunny Mode: Stance, and holding a Full Charge for 1 second or more switches her to Engage. In Stance, Enhance Bullet greatly raises Crit Rate and Critical Damage, and in Engage, Noise Bullet turns her normal attacks and Swift Carrot into DEF-ignoring damage. Normally, each Full Charge raises the normal attack damage multiplier and Charge Damage of the next 1 shot.

Using Burst switches her weapon for 5 seconds to Swift Carrot, a rapid-fire weapon with a fixed 0.5-second charge, where most of her firepower goes in, and at the same time her own ATK rises for 5 seconds. Her Burst Skill deals all-enemy damage plus, depending on mode, extra damage or DEF-ignoring damage to the same enemy. If she is in the same squad as Guilty, one's mode switch also applies to the other.

Right after battle starts, hold a Full Charge for 1 second or more to switch to the mode you want, then leave her on auto. Use Engage in DEF-ignoring squads and Stance in crit buffer squads.
Arena is fully automatic, so she only fights in Stance.

When using her in Stance without Guilty, put her in crit buffer squads with Mint, Prika, or Helm with her Favorite Item.

Most of her firepower comes from normal attacks, so use the Resilience Cube, and the Destruction Cube against bosses with tough parts.

Her magazine refills when switching to and back from Swift Carrot, so she barely needs Max Ammo. Fill leftover slots with Charge Speed or crit lines.

The Full Charge multiplier increase and Increased Charge Damage are on S1, so unlike Guilty, raise S1 first. S2 holds per-mode effects and Swift Carrot damage, so raise it next, and raise the Burst when you need all-enemy damage.

Used with Guilty in DEF-ignoring squads in Solo Raid boss fights against Fire enemies (= Water advantage). Her all-enemy Burst also makes her useful for clearing mobs in stages and the Missilis Tower.

She has an all-enemy Burst, so she is more versatile than Guilty.
She fits both Stance crit squads and Engage DEF-ignoring squads.
Her magazine refills when switching to Swift Carrot, so she needs little ammo investment.

Her firepower in Stance alone is lower than expected.
She cannot use Engage in fully automatic Arena.
If Guilty in the same squad holds a charge for long, Sin's mode changes too.