The Sylph summon consists of several pixies that steal HP from the enemy and give it to the characters. In FF4, it only gave HP to the caster; in FF5, the entire party. In Tactics, Silf did not steal HP, but instead looks like a little nymph/gnome creature and silences enemies. In FF12RW, Sylph is non-elemental (despite having an attack called Aero).