Hold the button to gather strength, then release to unleash a powerful magic attack. Affinity changes with ninjutsu. Can be used while airborne.
Description
Foxflame is a synergy skill in Final Fantasy VII Rebirth, used by Yuffie Kisaragi with either Aerith Gainsborough, Red XIII, or Cait Sith. It is available in her folio. The skill enables Yuffie to charge a strong magic attack before unleashing it, its element depending on Yuffie's Elemental Ninjutsu.
In the ability's animation, Yuffie enters a pose before her partner empowers her. This is then charged through her partner, before she unleashes a magic projectile at the target.
Yuffie has another ability, Shuriken Sync, with other party members.
Obtained
Foxflame is unlocked on Yuffie's folio for 10 SP.
Use
Foxflame can unleash significant magic damage, particularly considering it does not consume ATB, and has great range. However, it is a slow ability and renders Yuffie vulnerable while charging it, during which it will interrupt a party member. As a magic ability, Foxflame is based on Yuffie's 
Magic Attack, which for her is often prioritized in many builds, and is naturally decent given Yuffie's balanced attributes.
In certain circumstances, such as against slow or staggered enemies when Yuffie is low on ATB, or is under 
Slow, it can be useful. Given that Yuffie's playstyle is based around quickly building and using ATB charges, she will rarely be in this position, but if she is, it serves as a great source of damage.
Foxflame is different to Shuriken Sync, where Foxflame deals stronger magic damage than a basic attack, while Shuriken Sync is more similar to Yuffie's regular attack.
Etymology
A will-o'-the-wisp are atmospheric ghost lights seen by travellers at night, especially over bogs, swamps or marshes. It resembles a flickering lamp and is said to recede if approached, drawing travellers from the safe paths. The phenomenon is known by a variety of names, including jack-o'-lantern, friars's lantern, hinkypunk, and hobby lantern in English folk belief, and is well attested in European folklore.