Kujou Sara is a quickswap support who is able to buff your active character via her Crowfeather mechanic while dealing a decent amount of personal damage. When Kujou Sara uses her Skill, her next Charged Shot will leave a Crowfeather at the target location which explodes after a short while, granting the active character in its AoE an attack buff based on Kujou Sara's Base ATK. Active characters within her Burst's AoE will also receive the same buff or have its duration refreshed.
If building Kujou Sara for personal damage, it is recommended to let her receive the buff provided by her Skill to enhance her Burst, while letting the next active character be buffed by her Burst.
At [C2], you can skip the Charged Shot as her Skill immediately drops a Crowfeather, and save it for later on if you wanna swap back to her to extend her buffs. The CRIT DMG buff provided by Kujou Sara's [C6] cannot be snapshot and is not shown in the attribute page.
For her Skill to generate particles, you need to perform a Charged Shot to place a Crowfeather after casting her Skill. If the Crowfeather explosion hits an enemy, it generates 3 particles.
Crowfeathers dropped through Kujou Sara's [C2] do NOT generate particles.
Regarding Weapon Choices:
Weapons here are ranked based on Team DPS to account for Kujou Sara's Buff and Personal Damage.
Skyward Harp: This weapon combines the highest base attack for Kujou Sara's buff with good stats for her personal damage, making it the best overall pick.
Polar Star: This weapon trades some buff strength for more personal damage. Can pull ahead of if a quick rotation is performed to maximize its passive.
Elegy for the End: While providing a strong buff for the team and a large amount of Energy Recharge, Kujou Sara can have issues with reliably activating the passive in Single Target scenarios against small or mobile enemies.
Mouun's Moon: At [R5], this weapon performs similar to or better than , depending on your team's total Burst cost.
Alley Hunter: Kujou Sara's quickswap playstyle lets her take advantage of this weapon's passive well. Performs similar to at [R5].
Favonius Warbow: While being bad for both Kujou Sara's buff and personal damage, this weapon can be a good pick if the team struggles with high Energy Recharge requirements.
Regarding Artifact Choices:
Sets
Noblesse Oblige (4): A good pick for Kujou Sara if there is no other possible holder on the team, or if it allows for another buffing set to be added to the team.
Substats
Energy Recharge: Prioritize until requirements are met.