Can I use Discipline as a reaction to Fiora's Peerless ability?

Whether you can use Discipline in response to Fiora, Peerless depends on when you are trying to play it relative to her triggered ability.

According to the Showdown timing rules and RiftJudge FAQ #8421, here is the breakdown of the interaction:

The Timing Sequence

1. Designation: Fiora is designated as the attacker or defender in a one-on-one combat. Her triggered ability ("When I attack or defend...") is placed on the Initial Chain.
2. State is CLOSED: Because a triggered ability is on the chain, the game state is Closed.
3. Playing Discipline as a [Reaction]: Because Discipline has the [Reaction] keyword, you can play it while the chain is Closed.
4. Chain Resolution (LIFO - Last-In, First-Out):
   • Discipline resolves first: Fiora receives +2 Might.
   • Fiora's ability resolves second: Her Might is doubled. Because her base Might was increased by Discipline before her ability resolved, she will end up with higher total Might than if you had waited.

Important Distinction

• You CAN use Discipline in response to the trigger: Because Discipline is a [Reaction], it is allowed in a Closed State.
• You CANNOT use [Action] cards: If you were trying to play a card that is only an [Action] (i.e., it lacks the [Reaction] keyword), you would have to wait until after Fiora's trigger resolves and the chain empties, at which point you would gain Focus (Open State) and could then play the Action.

Summary: Yes, you can play Discipline as a [Reaction] to Fiora, Peerless's trigger. Since it resolves before her doubling trigger, your +2 Might bonus from Discipline will be included in the doubling calculation.