Do Star Crossed require me to return my unit to hand or can I choose only enemy unit?

You must target both a friendly unit and an enemy unit to play Star-Crossed. You cannot choose to target only an enemy unit.

According to RiftJudge FAQ #9865 and FAQ #9452, Star-Crossed requires both a valid friendly unit and a valid enemy unit to be present on the board to be played. Because the spell lacks "up to" language, both are mandatory targets. If you do not have a friendly unit on the board, you cannot fulfill the targeting requirements and therefore cannot play the spell.

If, after the spell has been played and placed on the chain, one of the targeted units becomes unavailable (e.g., if it is moved to the trash or back to a hand by another effect), the spell will still resolve on the remaining legal target, as established in RiftJudge FAQ #10058. However, you must have both targets available at the moment you initiate the play of the card.