Quick Answer
Fiona is a flexible Mystic support built around repeated Shield casts. The material says she had healthy win rates across all modes, and Version 0.5 identified The Lustrous as a standout in SSS and Endless before later tuning. She can be an item-and-Shard economy engine, Shield/Burn engine, or team Max-HP support.
How Vault Spark Works
Vault Spark applies Shields to low-current-HP heroes, with Shield size scaling from Magic and the number of targets increasing with Mana Regen in the current-looking material. Mana Regen is therefore one of Fiona's most universal stats. Magic gives bigger Shields, while Attack Speed can indirectly help by generating Mana through attacks.
The simplest rule is: Magic means bigger Shields, and Mana Regen means more casts, more targets, and more specialization synergy. Starting Mana can also be strong because it helps Fiona cast earlier in dangerous openings.
Specializations
The Right Hand creates a random item and gains Shards every 5 casts in the material. It is the best-defined Fiona economy and item-generation route. It pairs naturally with item-hungry plans, including Niklas.
The Lustrous is the high-difficulty Shield/Burn route. The material gives it the strongest official SSS and Endless signal, though it was nerfed because of that performance. The Tycoon is the long-run team Max-HP and Backup scaling route built around repeated casts and Mana Regen.
Build Priorities
Fiona wants Mana Regen, Magic, Starting Mana, Attack Speed, and sometimes Max HP for Lustrous. Do not publish a fixed best item list without current in-game verification. Build her around the role the team needs: economy, Shield/Burn combat, or Backup HP support.