The eladrins are the native race of Arborea, just as the baatezu are associated with Baator and the tanar'ri with the Abyss. They're wild and free beings who exult in their own existence and live a life of song and celebration. The eladrins aid all people of good hearts against the forces of evil, but seek to do so with individual acts of kindness or heroism.
In Arborea, the eladrins move from place to place constantly, reveling in the natural beauty or the plane and seeking adventure. They're defenders of goodness and freedom wherever it is threatened, and seek to counter the influences of tanar'ri and baatezu among mortals. To the eladrins, mortals should be free to choose their own destinies without fiendish interference: many of the more powerful eladrins constantly roam the planes and prime-material worlds, working against the baatezu and tanar'ri who seek to dominate these realms.
Note: Eladrins originally appeared in this form in the Planescape Monstrous Compendium Appendix II for the AD&D game from TSR. Their use here is for the purposes of providing context for the campaign only.