Get the quest "Showdown At Stonewatch" in Redridge Mountains, which you'll know by this stage of the questline gives you five NPC's, "Bravo Zero Company" (B.0. geddit?). This is a minimum level 17 quest designed for 20's.
Their leader is John J Keeshan a level 25 elite modelled on Rambo. All the NPC's are supposed to despawn if you leave Redridge Mountains. However, when logging out for a while and logging back in, it seems that on leaving the zone only four despawn and Keeshan continues to follow you. This is somewhat unreliable but seems to occur about 50% of the time.
Keeshan will follow you into Elwyn Forest (fun for duels), Westfall, and Duskwood, where he's pretty useful as a tank for levelling and will kill anything there. He despawned in other high-level areas, but he would likely get one-shot there anyway.
If you want to power-level try this: get the "Saving Foreman Oslow" Redridge quest, which is independent of the Keeshan questline, and acquire an Ettin with the control orb. Run through the Keeshan questline till you get to aforementioned Showdown quest. You now have five NPC's and an Ettin with a freaking tree as a weapon, and can trash all the Redridge mobs instantly till you've outgrown the area. Then take Keeshan with you to Duskwood.
Additionally if you like pvp one of the Bravo NPC's gives you a 2500 health buff. This lasts only two minutes but can make warsong gulch enormous fun right at the start if you time it right.