Talk to Aliza, who is standing near a tree across the road from Outskirt Stable. She wants to see the Master Sword. If you haven’t done so yet, this is your cue to go complete the main quest “The Hero’s Sword”, which you can learn about here.
Once she has seen the Master Sword, Aliza will give you a Star Fragment as a gift. If you have already obtained the Master Sword when you first see her, the quest will autocomplete and she will immediately give you the Star Fragment.