Though she is not a major character in Slayers, Sylphiel was blessed with her own image song sung by her talented seiyuu Touma Yumi as well as a BGM. I'm currently hosting mp3s of them so that they will be available, but the moment I find out they're being direct-linked from another site, I will take them down.
Alone (from Slayers NEXT Sound Bible III)
Alone starts out as a soft, almost wistful tune reminiscent of a music box and then leads up to a more upbeat rhythm that is very catchy, though the general tone is slightly sad. The lyrics are quite moving as they reflect Sylphiel's love for Gourry and how she matures in the series and has accepted that he is in love with someone else. This is possibly my favorite image song from the tv series.
Priestess Sylphiel (from Slayers Etcetora 2 Take a Chance)
Lyrics (not applicable)
In Slayers NEXT, you will hear Sylphiel's background music playing at certain scenes. It is a gentle and soft melody that is usually played when the characters take a moment to reflect or during significant moments that are supposed to be considered romantic. An example of when this theme plays is in Episode 42 when Amelia decides to give the Brass Rackets trophy to Zelgadis.