Man, I'm more active than the moderators on this board...
The Sylph can be found in the MM2.
To sum her up quickly, she's a Small Outsider (Air), she has 3 HD (advancement until 9HD, no size increase), a 30 ft. move, a 90 ft. (perfect) fly speed, SR 14, no particular natural weapon (and don't carry weapons usually). She's CR 5, she cast arcane spells as a sorcerer of her HD+4 in level,
improved invisibility at will, and
summon elemental (Large elemental, any element) once per day as a
summon monster VI spell cast at 12h level. Her average ability scores are Str 8, Dex 13, Con 8, Int 15, Wis 16 and Cha 17. She has the empower spell feat by default, and she has also a rather cute illo (click on the picture for larger version):
