Can you immediately cast Reaction without focus as defender in Showdown start while attacker has focus?

No, you cannot.

According to Rule 313.4, "A player may not make discretionary actions with Focus unless they also possess Priority." Furthermore, as established in the RiftJudge FAQ (id 9012), the attacker always gains focus first at the start of a showdown.

Because the attacker holds focus and priority, they are the only player allowed to start a chain. As the defender, you cannot act until the attacker either plays a card (starting a chain) or passes their focus to you.

Why you cannot act immediately:

1. Priority is Exclusive: Only one player holds priority at a time, allowing them to act. You cannot "pre-cast" a reaction or interrupt the attacker before they have made a decision.
2. Order of Operations:
   • Showdown Starts: The attacker gains both focus and priority.
   • Attacker's Choice: The attacker must either play an [ACTION] or [REACTION] card to start a chain, or pass their focus.
   • Your Turn: Only once the attacker plays a card (opening a Closed State) or passes focus to you, can you then use your own [REACTION] cards to respond or act.

In short, you must wait for the attacker to yield priority before you have the legal right to play any card, including those with the [REACTION] keyword.