Overview
Furina is an off-field Hydro Sub-DPS and Support, scaling on HP and relying on team HP fluctuations to access higher buffs and personal damage. She is directly able to drain teammate HP through her Elemental Skill, and while her Elemental Burst is active, she can track teammate HP gained or lost and turn it into a stacking resource called Fanfare. She cannot heal her team on her own, meaning she requires strong healing in her teams. She can switch between Ousia and Pneuma modes using her Charged Attack. These different modes modify how her Elemental Skill works, with Pneuma being generally useless before C6.
Elemental Skill & HP Drain
Furina’s Elemental Skill in her default Ousia mode spawns 3 ‘pets’ that follow the active character. These pets will launch attacks at enemies at different intervals. Surintendante Chevalmarin will attack roughly every 1.7 seconds, Gentilhomme Usher every 3.2 seconds, and Mademoiselle Crabaletta every 5.2 seconds. Their attack interval resets whenever Furina casts her Elemental Skill again. In Pneuma mode, it will spawn the Singer of Many Waters, that heals the active character every 2 seconds. Don’t use Pneuma mode. Furina is able to quickly Dash cancel her Elemental Skill’s animation endlag, potentially saving a bit of rotation time.
Whenever one of her Salon Solitaire members attacks, Furina will drain a percentage of all of her teammates’ health. Surintendante Chevalmarin drains 1.6% Max HP, Gentilhomme Usher drains 2.4% Max HP, and Crabaletta drains 3.2% Max HP. However, if a teammate is at 50% or less health, they will not have their individual HP drained. For every teammate whose HP was able to be drained on the pet’s attack, that attack will gain a direct multiplier of 10%, up to 40% if all 4 party members had their HP successfully drained.
The Salon Solitaire will stay on the field for up to 30 seconds, with subsequent Skill casts resetting the duration. Furina’s Elemental Skill is on a 20 second cooldown, and Furina will generate 1 Hydro particle on a successful Salon Solitaire hit on a 2.5 second cooldown. Due to the irregular intervals at which the pets operate, it will typically be about 8 particles per 20 second rotation.
Elemental Burst & Ascension 1
Furina’s Elemental Burst costs 60 Energy on a 15 second cooldown. After the animation plays out and deals a small AoE Hydro hit, it will immediately begin tracking the HP fluctuations of all party members. For every 1% of HP a party member loses or heals, Furina will generate 1 ‘Fanfare’, up to a maximum of 300 at C0. Each point of Fanfare will convert to 0.25% DMG Bonus (at talent level 10), meaning a maximum of 75% at max Fanfare.
At C0, the general expectation is that the off-field party members will have roughly 40% DMG Bonus on average from Furina’s Burst, and the on-fielder will have 50% DMG Bonus, although this is dependent on the team and rotation. This is a side-effect of having had more time to generate Fanfare stacks for on-fielders, assuming Furina goes early in the rotation. Fanfare generated will also give teamwide Incoming Healing Bonus to the team, 1% for every 10 Fanfare, directly increasing healing done and thus subsequently increasing Fanfare generation for Furina.
When a teammate is healed to full health, Furina’s Ascension 1 passive will heal the team for 2% of their Max HP on a 2s interval, for a total of 3 healing procs over 4 seconds. This can help with teams where healing is lob-sided, such as with Neuvillette’s strong self-healing resulting in a bit of team-wide healing or Escoffier’s single-target healing ticks. Furina’s Ascension 4 will give her Salon Solitaire hits DMG Bonus, up to 28% at 40000 HP.
Furina
Furina has taken on a role of a relatively niche Support and Sub-DPS for specific teams, having fallen far from her days of ubiquity. Featuring team-wide DMG% buffs alongside decent off-field HP-scaling Hydro damage, she remains strong in her niches. 
Skirk teams alongside 
Escoffier, and in general any Freeze team with Escoffier. These two form a very effective core of decent off-field damage and team-wide buffing that many Cryo carries synergize greatly with. She does have some limited flexibility in other teams that require off-field Hydro application, although the abundance of those is a thing of the past. 
Scroll of the Hero of Cinder City and 
Celestial Gift, or no use for them at all like any Lunar or Stellar team. Furina also has a second form for her Elemental Skill which heals the active character without doing anything else – this is generally not used besides minor gameplay optimizations. Although she’s not what she used to be, she is capable in select situations, particularly if you happen to have some of her early Constellations.
Sandrone
Columbina