Overview & Draconic Glory
Neuvillette is an HP-scaling on-field DPS based around his Enhanced Charged Attack. He has large AoE and is a relatively maneuverable and dynamic character in terms of gameplay, but lacks frontloaded damage and can have less rotational flexibility than most. Neuvillette's Ascension 1 Passive will give him 'Draconic Glory' stacks, which multiply the damage done by his Enhanced CA based on the amount of unique Hydro Reactions triggered in his team. Neuvillette will deal 110 / 125 / 160% of his Enhanced CA's original damage, corresponding to 1 / 2 /3 unique Hydro Reactions triggred. Because Neuvillette typically requires additional Energy from his teammates and has natural synergy with Hydro supports like 
Furina, he will typically want to only go up to 2 of those stacks. His C1 will naturally provide him with an extra stack though.
Enhanced Charged Attack & Sourcewater Droplets
In order to access his Enhanced CA, Neuvillette has to charge it for a few seconds. He can reduce this charge-up time by absorbing 'Sourcewater Droplets', a mechanic he and a few other Fontaine units have. Sourcewater Droplets are generated when Neuvillette's Skill hits an enemy and when Neuvillette casts his Burst, as well as minimally by 
Sigewinne and 
Traveler (Hydro), although they are not good Neuvillette teammates. Any Sourcewater Droplets generated will last for 15 seconds, meaning Neuvillette has to make conscious use of them.
When Neuvillette starts up his Enhanced CA Charge, he will begin absorbing nearby Sourcewater Droplets within his field of vision. This means he won't absorb them if they're off-camera. Each Droplet absorbed will reduce the required time to complete the Charge and begin the Enhanced CA. If Neuvillette absorbs 3 Droplets, he will begin it near instantly. His Skill generates 3 Droplets, whereas his Burst generate 6.
Enhanced CA Details & HP Fluctuation
Charged Attack: Equitable Judgement is a Hydro laser with very large AoE. It will hit all opponents caught in its AoE throughout its duration of just above 3 seconds. During Equitable Judgement, Neuvillette can freely move around and aim it, and he will not automatically target enemies. Neuvillette can freely let go of his CA input while the animation is on-going, and it will not be cancelled; to do that, he has to input a Skill, Burst, or Dash out of it.
Equitable Judgement has a total of 8 hits it will perform throughout its duration. During its duration, Neuvillette will also periodically drain his own HP, at a rate of 8% max HP every 0.5 seconds. Whenever Neuvillette absorbs a Sourcewater Droplet though, he will restore 16% of his max HP. This makes Neuvillette's HP highly fluctuating, which ties into his Ascension 4 Passive. This will give him some extra Hydro DMG Bonus based on the amount of HP he is currently at. This will max out at 30% Hydro DMG Bonus at 80% of his Max HP.
Neuvillette's HP fluctuation also makes him a great teammate for Furina, whose Burst will record changes of her teammates' HP percentage. Neuvillette is one of the few characters in the game that can comfortably use Furina without a dedicated healer, although having one doesn't necessarily hurt.
Elemental Skill & Elemental Burst
Neuvillette's Elemental Skill will deal a little bit of AoE Hydro DMG on cast. If it hits an enemy, it will generate 4 Hydro Particles and spawn 3 Sourcewater Droplets. This Skill has a cooldown of 12 seconds, and Neuvillette will typically always want to perform 2 of them per rotation. Because of that, Neuvillette's best rotations will typically be quite long, at roughly the duration of 2 of those cooldowns + 1 Charged Attack duration. This is mostly done for Energy purposes, as Neuvillette has a pretty hard time getting his Elemental Burst back otherwise.
Neuvillette's Burst has an 18 second cooldown and Energy cost of 70. On cast, it will deal 3 hits of AoE Hydro damage and spawn 6 Sourcewater Droplets. Neuvillette will want to use this Burst every rotation, as it gives him an extra 2 Enhanced CAs.
Enhanced CAs and Rotations
With his 2 Skill casts and 1 Burst cast per rotation, Neuvillette will want to perform a total of 4 of his Charged Attack: Equitable Judgement. Each of these will last for about 3 seconds, so Neuvillette's total field time will be about 16-17 seconds. This is quite long, but because Neuvillette's rotations will typically run at a minimum of 26 seconds, he will have time to spare for his teammates.
This means that besides teammates with very high uptime like Furina or Columbina, Neuvillette also works well with anyone who can easily refresh their buffs. Neuvillette will actively want his teammates to do this, in order to maintain buff uptime and fill out rotational downtime.
Neuvillette
Neuvillette is an HP-scaling on-field Hydro DPS. Through his unique HP fluctuation mechanics and his passive asking for non-Hydro elements in the team, he has relatively unique teambuilding. The unequivocal best on-field DPS at a time, he has since fallen quite far behind the meta curve. Although still able to perform at the top endgame echelons with some early vertical pickups, his weaknesses at lower investment have been laid bare in Stygian Onslaught. Often-times reliant on some newer supports like 
Ineffa, his endgame flexibility will no longer cut it for several bosses. Worth noting that despite his passive encouraging Rainbow teams, Neuvillette will essentially always want to play with a 2nd Hydro unit in his teams for Energy reasons. 
Tome of the Eternal Flow are even more important compared to before if looking to keep up with the increasing difficulty of newer endgame. Lunar units in particular can let him punch above his weight, and he will often not compete with newer units without them against many endgame scenarios. Neuvillette's rotations also tend to be very long and place constraint on how units without the ability to sustain significant uptime. This, combined with his lack of frontloaded damage, makes him ill-suited to be competitive a lot of the time.
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