Charlotte Corday

Charlotte's skills all help with her performance one way or another, although the skill that has the best scaling per stat level is probably her third, despite it having built-in randomness. Maxing her skills is not the highest priority, although if Masters are planning to use her often, then higher levels in all her skills are not unwelcome.

• Angel of Assassination A – This skill is her survival, which does not change with skill level aside from its cooldown (which is still appreciated). The Death Rate buff increases with level, up to +100%, and if looking to her in need of an Instant Death Servant to handle a fight gimmick or similar, putting a couple levels into her first skill at low cost to stave off RNG wouldn't be too much given the low investment costs for a 1 star Servant.
• Iron Determination D – Levels in this skill will boost the ATK buff as well as the amount of Debuff Resist provided. The Ignore Invincible effect is unaffected by level aside from cooldown, and the ATK buff unfortunately only lasts for one turn, making this secondary in priority for leveling, although still useful for increasing her burst damage.
• Haphazard Planning D+ – Raising this skill will provide boosts to all three of the random effects that it may provide, all at decent values per level, so that regardless of which buff it randomly gives you, they will all benefit from leveling this skill. All buffs this skill provides also last three turns, making investment in this skill probably the best return for cost. Level this skill first, particularly if planning to run Charlotte Corday as a damage dealer.

“Though I cannot move the word, neither can the world slow me down. I pray I am forgiven. Please accompany me in this unknowing murder.”

Charlotte Corday, famous for the assassination of Jean-Paul Marat during the French Revolution, manifests befittingly as the Assassin of the 4th Anniversary's Bronze Servant batch. Her kit is decidedly offense-oriented, with her being a single-target Arts damage dealer with instant death effects.

Charlotte's skill kit is largely selfish, with the most interesting skill in her kit being Haphazard Planning, which provides one random effect of either Arts, Crit Damage, or NP Damage buffs, all at decent values. She also has a self attack buff that grants her a turn of Ignore Invincible and increased Debuff Resist for three turns from Iron Determination. Finally, Angel of Assassination is her primary defensive skill, providing a 2-time evade for 3 turns, while also boosting her Instant Death success rate buff for a turn. The Instant Death buff complements her Arts NP, Le Rêve Ensoleillé, which will remove Evasion buffs before dealing DEF-ignoring damage, and a chance to inflict Instant Death scaling with overcharge. 

Her Arts/Quick/Extra cards have decent hit counts, giving her decent NP gain off an AQA chain, although her NP itself only has 1-hit so it is not a strong candidate for NP looping. Her chance-based third skill can also make her performance somewhat unpredictable, although it should be noted that all three effects are decent. Her low rarity means her stat caps are much lower than most Servants, which unfortunately hampers her performance as a whole. 

Overall, Charlotte Corday fills out the bronze Servant Assassin roster with a new addition to the ST Arts Assassin list, and she performs decently enough to fill the role of a Servant capable of inflicting Instant Death if the situation calls, while dealing some pain to Rider enemies here and there. And if Masters opt to invest in her, they will be rewarded with a smile from her small angelic egg companion.

Ignore Invincible on Skill

Having access to Ignore Invincible on demand, Charlotte Corday can essentially ignore any Evade or Invincibility skills for a turn if the situation arises. Evade/Invincibility periodically appears on Rider class enemies, particularly Servants, for which Charlotte Corday would most likely be used, so this skill may come in handy decently often. 

High Instant Death Rate

Charlotte Corday is a relatively easy-access Servant available in the Friend Point summoning pool with a notably high Instant Death rate, allowing her to fill in for this role for Masters with underdeveloped rosters if an encounter calls for it. Her death rate of 160% at skill level 10 without overcharge is well above average for instant death effects, only beaten by Servants such as First Hassan and Ryougi (Assassin). This effect is still quite niche, however, given many bosses are entirely immune to instant death, but is still useful to have.

Low Rarity Stats

As with most Bronze Servants, Charlotte Corday's performance ceiling is somewhat hampered by her Bronze Servant status, which naturally will come with lower stat caps, even with grailing. Both her HP and ATK stat are low, making her damage output somewhat low while also making it a bit more difficult to keep her on the field.

Random Buffs on Third Skill

Her third skill will randomly select one of three buffs, all of which last three turns. The Arts buff is arguably the most effective one, given it will affect the NP Gain and damage of her Arts Cards and NP. The NP Strength buff provides the largest boost to her NP Damage, although its three turn effect may be difficult to take advantage of due to her lower NP refund without the help of supports. Finally, the Critical Strength buff has decent value, although she lacks any instant star generation skills or star gather increase buffs, so it can be somewhat difficult to fully take advantage of this buff as well. 

Charlotte benefits from Craft Essences that can either provide additional NP Gauge, NP Damage, or Arts Performance buffs, if building her offensively. Given Charlotte has no source of instant NP charge herself, starting NP Gauge will go a long way in helping to give her access to her NP more quickly. 

• The Black Grail: As per usual, The Black Grail provides the largest raw damage boost for her  NP.
• Kaleidoscope / The Imaginary Element: These provide some measure of starting NP Gauge, which will help Charlotte Corday ready her assassination more quickly.
• Painting Summer / Grand Puppeteer / New Beginnings / Dive to Blue / Schwipsig in the Snow / Chaldea Anniversary: These CEs provide a hybrid of starting NP and an Arts buff, which are useful for boosting her overall offensive performance, including her NP Gain on her Arts Cards as well as their damage.
• Holy Night Supper / Golden Sumo: Boulder Tournament: These provide starting NP and an assorted offensive buff, which is also helpful to boosting her NP damage, although these do not affect her NP gain.