Genshin Impact's Furina has an admittedly confusing kit on the surface level; there's a lot to read and understand, and it might overwhelm some when first looking at it. She can change between Fontaine's two Arkhes, Ousia and Pneuma, to be either a Hydro support or a healer. Her default mode is Ousia and this is the most useful mode to players. She drains HP from her teammates when using her skill in this Arkhe mode though, so players should be mindful of what healer they run alongside her.
Furina's Pneuma form can only heal the active character, does not regenerate energy for herself or others, and is a general loss for her potential as a Hydro unit. She switches between the Genshin Impact Arkhe modes with her charged attacks, but players shouldn't switch out of her default Ousia, since that mode is where her true battle utility lies. In this mode, she is capable of buffing all party members' damage, dealing decent Hydro application, and using her summons to bolster the team as a whole.
Furina's best builds will have her using a 4pc Golden Troupe set for its massive 75% elemental skill damage bonus. Her main point of damage and Hydro application comes from her skill, which makes this set ideal for her. No other set can compete with this, but for players who have yet to get the ideal set for her, a 2pc Golden Troupe with mix-and-match sets for either Hydro damage bonus, HP, or energy recharge can work in the meantime. Her ideal sub-stats will be HP, energy recharge, and both Crit Rate and Crit DMG.
It is worth mentioning that the Sumeru set, Vourukasha's Glow, does not work on her despite the skill bonus. The drain of HP her kit inflicts does not count as damage and renders the set useless on her.
Unfortunately, Genshin Impact's Furina's weapons can be extremely limited for new players. Besides her signature weapon, one of her best free-to-play weapons is Festering Desire, a weapon last seen in version 1.2. The next best free-to-play weapon can be annoying to obtain - the Fleuve Cendre's Ferryman, obtainable via fishing. There's a great number of five-star weapons that will work wonders on her in comparison, though, such as:
As a Hydro unit, Furina can work amazingly in reaction comps. She isn't ideal for too many Hypercarry situations, but being Hydro compared to any other element works in her favor. Due to her energy costs, it is recommended that she run alongside another of Genshin Impact's Hydro units so that she can generate enough energy to make use of the fact that her burst has a potential 100% uptime. Units like Xingqiu or Yelan work well alongside her.
She's going to need a healer who heals more than just a single target. While playable characters like Bennett or Kuki Shinobu are amazing, they do not synergize with Furina and would need to be run alongside yet another healer. However, if a player chose to use Kuki and Baizhu, this could make a solid team comp for Furina. She is flexible and can run almost anywhere, but she has to have a healer on the team because of her draining HP mechanic.
Genshin Impact's Furina's constellations are wonderful additions to her kit. Though costly, they are a good investment that players should make if they can. Her best constellations are her C2 and her C6. Both of these massively improve her utility and her use of the fanfare stacks gained in her burst. Her fanfare points are what make her a good support, boosting the party member's damage. Her C2 makes her max out that potential quicker, and her C6 infuses her attacks with Hydro and adds an entirely new aspect to her kit, making her a hypercarry.