If Fiora gains Deflect during a chain (after an ability targeting her is already on the chain), does the player have to pay the Deflect cost when resolving that ability?

Ruling: No, you do not pay the Deflect cost. Deflect is paid as an additional cost when the spell/ability is put onto the chain, so if Fiora gains Deflect after an ability is already on the chain with costs paid, you won't retroactively modify those costs.

Sequence:

• Charm is played targeting Fiora and put on the chain with all costs paid
• Opponent responds with Call to Glory, making Fiora mighty (giving her Deflect)
• Chain resolves without retroactively adding Deflect costs to Charm