Does Mindsplitter trigger and reveal hand if I use Star-Crossed on it in response to the trigger?

Yes, Mindsplitter’s "When you play me" ability will still resolve and force your opponent to reveal their hand, even if you use Star-Crossed in response to return the Mindsplitter to its owner's hand.

Why this works:

1. Triggered Abilities Exist Independently: Once a triggered ability is placed on the chain, it exists independently of its source. Even if the Mindsplitter is returned to the hand by your Star-Crossed, the ability remains on the chain and will resolve normally.
2. Referents Checked on Resolution: While your Star-Crossed removes the Mindsplitter from the board, the Mindsplitter's ability does not require the unit to remain on the battlefield to resolve its effect. It simply requires you to have chosen an opponent when the ability was placed on the chain.

Sequence of Events:

1. Mindsplitter enters: Your opponent plays Mindsplitter; it enters the battlefield immediately.
2. Triggered Ability: Mindsplitter’s "When you play me" ability triggers and is placed on the chain, creating a Closed State.
3. Your Reaction: You play Star-Crossed as a [Reaction], targeting the Mindsplitter and a friendly unit.
4. Chain State (Top to Bottom):
   • [Top] Star-Crossed
   • [Bottom] Mindsplitter's ability
5. Resolution (LIFO - Last-In, First-Out):
   • Star-Crossed resolves first: Both targeted units are returned to their owners' hands.
   • Mindsplitter's ability resolves second: Despite the Mindsplitter no longer being on the board, the ability resolves. Your opponent must reveal their hand and discard a card as instructed.

This is consistent with established game mechanics regarding triggered abilities and how they interact with removal spells like Star-Crossed (see RiftJudge FAQ #10299 and #10703).