Can not so fast be used on Star-Crossed?

Yes, you can use Not So Fast to counter an opponent's Star-Crossed.

Reasoning:

1. Targeting Relationship: According to RiftJudge FAQs #10769, #10927, and #9865, Star-Crossed creates a mandatory targeting relationship with both a friendly unit (the controller's) and an enemy unit (the opponent's).
2. Interaction with Not So Fast:
   • Not So Fast can counter an enemy spell that "chooses a friendly unit or gear" (relative to the player of Not So Fast).
   • When an opponent plays Star-Crossed, they must target one of your units as the "enemy unit." Because this target is a unit you control, it is a friendly unit to you.
   • Since the opponent's Star-Crossed targets a unit friendly to you, it meets the requirement specified in RiftJudge FAQ #8638 and #8733 for being a valid target for Not So Fast.
3. Timing: Star-Crossed is a [Reaction] spell, which creates a chain and puts the game into a Closed State. Since Not So Fast is also a [Reaction] spell, it can be played in response to Star-Crossed while the game is in this Closed State.

Resolution Sequence:

1. Opponent plays Star-Crossed: They declare their target (a unit they control) and your target (a unit you control). The spell is placed on the chain.
2. You play Not So Fast: You target the Star-Crossed on the chain.
3. Resolution: Because the chain follows Last-In, First-Out (LIFO) order (Rule 340.1), Not So Fast resolves first. It counters the Star-Crossed spell.
4. Effect: Per Rule 425.1.a, the countered Star-Crossed does nothing and is cleared from the chain. Your units remain on the battlefield.