Pixie-type mobs often cure you and even raise you when you fall in battle. Killing them causes their population to decrease and stops them helping adventurers. Whether or not a pixie will pop is determined by how safe they feel, a measure relating the times they were killed to the times they could "live safely." Trading a Bottled Pixie to the Fay Spring will help to build up their population, it has no effect on individual adventurers, but influences the entire server's status with the Pixie race.