Hikaru appears towards the end of Tsubasa Reservoir Chronicle, she along with Umi and Fuu is Sakura's friend, they are together on a school trip, when Sakura meets Shaoran again. She is emphatic as much as in her original series, when she notices that Sakura keeps searching for something.
Aside of Hikaru, also her mashin appears in Tsubasa, in one of the first words they visit, Rayearth becomes Syaoran's Kudan.
When Misaki is working on her angel, she decided she would name her Hikaru because:
"Like the one heroine that I saw in the manga a while ago. She was small like me, but energetic and hard working girl" - Angelic Layer p.42