how katarina reckless works? the hidden card needs to be in the same BF?

Let me give you the ruling based on what I've established so far, then verify one remaining point.

No — Katarina, Reckless's trigger is not restricted to the battlefield where the hidden card was. Her ability reads "When you play a card from face down, deal 2 to an enemy unit." It has no "here," so both the trigger condition and the damage target work globally.

Key points:

1. Trigger scope: Katarina's first ability ("When you hide a card, ready me") and second ability both lack location qualifiers. Unlike abilities that say "when you play a spell here" or "when I attack," her trigger fires on any card you play from facedown, at any battlefield. If the hidden card is at Battlefield A while Katarina sits at Battlefield B, the trigger still fires.

2. Damage target: The "deal 2 to an enemy unit" effect has no "here" or "at a battlefield" restriction — similar to FAQ #7344's general principle that, absent a location restriction, targets include units at Base or any battlefield (CR 355.9.b). You can target any enemy unit anywhere — Base included. If there's at least one enemy unit with Deflect, note the trigger is mandatory (no "may"), but you can choose to decline paying the Deflect cost and the ability fizzles (FAQ #11678).

3. Contrast with Hidden play restrictions: This is not a play effect of the hidden card itself — it's Katarina's own triggered ability. Rule 811.1.d.2's same-battlefield targeting lock applies only to the abilities of the card played from Hidden (e.g., Kennen's stun, per FAQ #11865). Katarina is a separate permanent on the board, so those restrictions don't apply to her trigger.

For example, Kennen's "When you play me, stun a unit" from Hidden is locked to his hiding battlefield because it's his play effect. Katarina's "When you play a card from facedown, deal 2" is a separate unit's reaction to the event — no location lock.