Guide
How the kit works
Before a fight, use Phantom Illusion to gain Stealth for 20 seconds. If Seele attacks an enemy to enter combat while Stealth is active, she enters Amplification and hits a random enemy for damage equal to her Skill multiplier. That hit is guaranteed to be a CRIT Hit.
In combat, Sheathed Blade deals Quantum DMG equal to 360% of Seele's ATK and raises her SPD by 25% for 3 turns. Its automatic attack can trigger once per turn and resets at the start of Seele's turn. It consumes no Skill Points and regenerates no Energy. If there is no valid target, it hits the enemy with the lowest HP percentage instead.
Use Butterfly Flurry for a hit equal to 720% of ATK. It also puts Seele in Amplification. Resurgence (Talent) gives her Amplification and an extra turn when her Basic ATK, Skill or Ultimate defeats an enemy. Amplification increases her DMG by 80% for 3 turns. A kill during the extra turn from Resurgence does not trigger another Resurgence.
Thwack (Basic) deals 100% of ATK as Quantum DMG, and a passive advances Seele's next action by 20% after she uses it. Other passives give her 25% Quantum RES PEN during Amplification and increase her DMG by 50% per kill, up to 3 stacks lasting 3 turns.
Build and why
For relics, start with 4-piece Genius of Brilliant Stars. It gives 10% Quantum DMG and ignores 10% of the target's DEF, with another 10% ignored against Quantum Weakness. Scholar Lost in Erudition is an alternative: it raises CRIT Rate and boosts Skill and Ultimate damage.
For planar ornaments, Rutilant Arena gives 8% CRIT Rate and, at 70% current CRIT Rate, increases Basic ATK and Skill damage by 20%. Firmament Frontline: Glamoth is another option: it gives ATK and more damage at 135 or 160 SPD. Check Seele's SPD in combat when judging those thresholds.
Use CRIT Rate or CRIT DMG on the body, SPD or ATK Percentage on the feet, Quantum DMG Boost or ATK Percentage on the sphere, and ATK Percentage on the rope. For substats, prioritize CRIT Rate, CRIT DMG, ATK Percentage, then SPD.
In the Night is the first Light Cone choice. It gives CRIT Rate and rewards SPD above 100 with Basic ATK and Skill damage plus Ultimate CRIT DMG. The Hell Where Ideals Burn is another option, with CRIT Rate and ATK after Skill uses; its combat-entry ATK bonus requires an allied Skill Points limit of at least 6. Cruising in the Stellar Sea gives CRIT Rate, an extra CRIT Rate bonus against enemies at 50% HP or less, and ATK after a kill.
Leveling priority
Follow the game's order: Butterfly Flurry, Sheathed Blade, Resurgence, then Thwack. The Ultimate has Seele's largest listed attack multiplier, while the Skill is used both on her turn and automatically after an ally attacks. The Talent raises the value of her Amplification damage bonus.
For maximum ascension, gather 308,000 Credit, 65 Void Cast Iron, 15 Thief's Instinct, 15 Usurper's Scheme and 15 Conqueror's Will.
To raise all skills, you also need 3,000,000 Credit, 8 Tracks of Destiny, 18 Arrow of the Beast Hunter, 69 Arrow of the Demon Slayer, 139 Arrow of the Starchaser, 12 Guardian's Lament, 41 Thief's Instinct, 56 Usurper's Scheme and 58 Conqueror's Will.
Eidolons worth it
For the biggest changes, consider Shattering Shambles first. After Seele's Ultimate, its target takes an extra instance of True DMG equal to 30% of her Ultimate DMG after receiving an attack. If any unit defeats that marked target, Seele's Talent also triggers. The mark lasts 3 turns and is removed if Seele is knocked down.
Extirpating Slash is another useful step: against enemies at 80% HP or less, Seele gains 15% CRIT Rate and ignores 20% DEF when dealing damage. Dancing Butterfly lets her Skill's SPD boost stack twice.
Flitting Phantasm gives 15 Energy on a kill, so it is worth considering if you want more Energy from defeats. Dazzling Tumult and Piercing Shards raise skill levels rather than adding new mechanics; you can treat them as lower priorities.
Teams
Sparkle is a Quantum buffer named for Seele by several guides. Cerydra is another buffer to consider. Their role is to support Seele while she concentrates on individual targets.
Silver Wolf is a Quantum debuffer, while Natasha can cover sustain. For example, Seele, Sparkle, Silver Wolf and Natasha give you a damage dealer, support and sustain in one team. Seele's automatic Skill gives the team another attack after an ally leaves a target at 50% HP or less.
Seele's kit does not state a required teammate element. Quantum Weakness on an enemy matters for the extra DEF ignore from Genius of Brilliant Stars, but it is separate from the elements you choose for the rest of the team.
Common mistakes
Don't expect an endless chain of extra turns. Resurgence (Talent) cannot trigger another Resurgence when Seele defeats an enemy during the extra turn it gave her. Use that turn with the remaining enemies in mind.
Don't count on every ally attack triggering Sheathed Blade. The target must be at 50% HP or less after the attack, and the automatic Skill has its own limit. It also gives no Energy.
Don't build around a planar threshold you haven't reached. Check current CRIT Rate for Rutilant Arena and SPD for Firmament Frontline: Glamoth. If you use In the Night, remember that its SPD scaling starts above 100, not at 100.