Sei Shounagon

Most Masters look to Sei for looping. This makes her The Stars' Clusters B skill crucial and what all other skills revolve around. Masters typically will pop at least her first and third skill at the first turn for continual NP gain, and her second skill can wait to the third if she needs to crit to clear the wave.

• Great Poet's Poem B is an attack boost with a crucial 10% per turn NP gain which helps Sei loop. Its heal generally is a wasted bonus but could be helpful in some situations. As an attack boost, this is best leveled second but can be safely left to level 9. 
• Meeting Checkpoint of Osaka A is Sei’s crit boost and a powerful evasion. At minimum she can evade all attacks against her for a turn, and even if one does not attack her she can keep her evasion for up to three turns. This skill does not help her loop so typically is left to last, and in pure loop setups is not necessary to level at all. 
• The Stars' Clusters B is what makes Sei so powerful. With a NP gain and a Quick boost, this enables her to loop without the need of a MLB Kaleidoscope. This skill scales both NP gain and Quick so should be leveled to max in most situations and first.

“Hehe! I luv it, I luv it, I super luv it!”

Who wants to party? Sei Shounagon does, that’s who! Her desire to become a huge partygoer spreads onto the field as well, making her a worthwhile AoE Quick Servant capable of looping with the proper setup. Her partying and double peace are sure to save the universe!

Most Masters will use Sei as a looper in a double Skadi Setup. With her charge per turn on Great Poet's Poem B and an on-demand charge of 20% on The Stars' Clusters B, along with an AoE Quick Noble Phantasm, she is capable of building 100% charge with a Waver and regular Kaleidoscope. She also is a uniquely talented critical damage dealer via the buffs from Meeting Checkpoint of Osaka A. Even better, she’s a durable Servant with her triple evasion on Meeting Checkpoint of Osaka. And it can’t go without saying, she has a wide coverage of supereffective damage on her Noble Phantasm, applicable against many types of Servants.

It is hard to find flaws with Sei, but there are a few around. For one, her Noble Phantasm isn’t the hardest hitting, especially in non-looping setups. Her skill’s effects also are grouped together sub-optimally, sometimes with a mix of defense and offense within the same skill, which may be useful at different times. Lastly, her Noble Phantasm’s supereffective traits are not entirely likely to occur together, so they almost never stack together for truly high damage.

Sei is an excellent Servant among a crowded AoE Archer field, especially if one has a Skadi and Waver to support her with. When Append Skills are available, it will be very helpful to have an extra 20% starting gauge on her second append skill, which can be unlocked with enough Bond levels ~Level 6 (or extra copies). But for now, if you need a party girl, look no further than Sei!

Looping

Sei is a looper. She is set up with two Skadis, a Kaleidoscope (regular or MLB), a Waver in the backline, and a Chaldea Combat Uniform. Using her third skill to boost her gauge to max, two Skadis on her, and setting her first skill’s per turn to go, she will be able to gain 19% gauge plus 10% on most all first waves. Boost her to 79% with a Skadi and swap in a Waver to boost her further to 100% with his attack and defense buffs. This will gain between 20-29% total, which can be topped off with Waver’s crit skill and the other Skadi’s 50% NP charge. Notably, she is unable to loop against Berserkers. 

Criticals

Sei usually is set up as a solo damage dealer with Caster supports so usually has no issue gathering Stars to herself. However, this is further boosted with her passive Independent Action (Self Centered) A which boosts her Quick gather rate another 10%. Her The Stars' Clusters skill which she usually uses turn 1 generates 10 stars on its own, and her Noble Phantasm generates a ton, so she’ll usually always have the Stars she needs to crit. These crits are then boosted by Meeting Checkpoint of Osaka making her a force to deal with even with face cards. 

Sturdy

Sei in general would not be a sturdy Servant with her low HP, but her Meeting Checkpoint of Osaka changes everything. Three hits for three turns is a massive amount of evasion, and even when she is hit her Great Poet's Poem will heal a decent amount each time. 

Bonus Damage

Sei’s Noble Phantasm is supereffective against three separate traits: Servants with Human attribute, Neutral alignment, and Shadow Servants. When two of these overlap she is just as effective as a Servant with class advantage, and against Sabers where she already has class advantage, the bonus is multiplicative.

Low Damage

Without supports, Sei only has a 20% attack and 30% Quick boost, leading her to have rather low strength on her Noble Phantasm. Her card damage is decent but nothing spectacular, and her NP gain without a Skadi support is putrid. She really needs supports to shine. 

Poor Skill Grouping

Sei is designed to have all her skills go off on turn 1, with the exception of maybe her second skill. The most painful of these is her The Stars' Clusters which she needs to use turn 1 to make use of her Quick looping, but burns a 20% charge. Her first skill can have problems too as it burns the heal the first two turns when the party most likely is not going to be hit. 

Supereffective Split

Sei has three potential supereffective modifiers: Human, Shadow Servant, and Neutral. In the game there are very limited Neutral Humans who are Saber or Berserker class, so she never fully can take advantage of this. In addition, most Servants are standard Servants outside of a few free quests in Singularities, so this is usually not applicable. 

Masters trying to loop with Sei currently need to have a Kaleidoscope. Lacking one, Masters will be looking to maximize her damage over two turns with a Mixed Starting Gauge / Damage CE, or to put everything into one big burst with a NP Strength Up CE.

• Kaleidoscope: Sei does not generate enough natural NP regeneration to use other CEs and loop. She will have to have a Kaleidoscope for any setup involving it.
• Knights of Marines / Holy Night Supper / Golden Sumo: Boulder Tournament: Once Servant Coins come out, Knight of the Marines at MLB becomes a powerful option for a NP2+ Sei. With 60% charge it allows her to use her third skill and append 2 to achieve 100% and benefit from its Quick boosts for all three turns (helping with regeneration). Until then, prioritize Holy Knight supper as she has no Noble Phantasm Damage boosting skills. 
• The Black Grail: If there is a situation where multiple of her Supereffectives hit, she has the potential to be one of the highest damaging Servants. Boosting this damage with a Black Grail turns her into a true damage dealer against Single Target even if she is AoE.