There are a number of NPCs that can follow you through your travels in Fallout 3. Normally, you are allowed to have Dogmeat and one other NPC from the following list as your follower. Temporary NPCs can be added for specific quests.
Other than having to pay for them, most NPCs have specific karmic requirements either to get them in the first place or for their own moral code - there are 2 NPCs for the 3 karmic paths (evil, neutral, good) and 2 more with no karmic requirements.
NPCs do not level up independently, instead they scale to the level of the PC. That is to say, the humanoids do, this effect does not seem to apply to Sergeant RL-3, Fawkes or Dogmeat. For all other NPCs, we have listed 2 separate hit point values, one for PC level 1 and one for PC level 20. For these NPCs, 3 skills (the "main skills") will rise as the PC rises in level. We have put in the value at level 1, the value at level 20 is 100 for all 3 skills. The non-humanoid NPCs also have main skills, 6 of them instead of 3, but they do not rise in value. Additionally, all humanoid NPCs have the ability to wear any armor including power armor, while the three non-humanoids can't wear any.
Each NPC (except Dogmeat) comes with a standard inventory with infinite ammo for their standard guns. These items are listed. On top of having infinite ammo, each NPC will regenerate all damage suffered during combat shortly after combat ends and no hostile enemies remain. If you need to heal an NPC during combat, simply speak to them and offer them a stimpak. Sergeant RL-3 can not be healed by Stimpaks, but has to be repaired instead.
