Shizuka Na Shibi is an instrumental background song to the anime xxxHolic. It was written and composed especially for the anime.
Even though some might think that because the title also contains the name of one of the anime's primary characters, Shizuka Doumeki, this song has no relation to the character. This song is usually played when two characters (usually Watanuki and Zashiki Warashi) have a kind and sweet moment together. Just like the scene it plays in, the song is not romantic but rather friendly and gentle.