I played Faendal as a follower in my last build - he is great! He used the armor and weapons I gave him and he is a good warrior.
Last night, we were clearing out a bandit cave (you know the one with bandits, a chief bandit and a chest

) and he was killed. No biggy, I'll tilde to consul and resurrect....it did not work!!!! I went over to one of the dead bandits, resurrected, they jumped up and started fighting again. I went back and tried to resurrect Faendal again....but he just layed there.
Maybe this was a Princess Bride effect, the bandit was only "Mostly Dead".
RIP my friend.
It happens. Faen is probably my favorite starter, so I can save Jenassa for later on without having to Wabbajack her. That's the thing, he's a low level follower, so at some point, you have to move on. Even if you do take him up to 30, he's just not that strong in the mid game.
So, what level are you, and have you been inside the Drunken Huntsman at all? Here's the trick, every NPC is leveled with you at whaterever you where when you first enter their cell. They're supposed to advance with you, they don't, there are fixes. You can manually set their level in the Console, or use the Wabbajack (there's other effects, but none so easy, and relatively glitch free) to advance them you your level (Just turn them into anything else, when they change back, they reset, but save first. We are talking about the Artifact of Sheogorath here, so it can, and will go horribly wrong.) Hopefully, you'll get a bunny, but it could be a Dremora Lord, Shower of Gold (that doesn't turn back) or just kill them outright.
Or, if you already nerfed her level, there's other decent choices like Belrand, Eola, Aela, and Mjoll. The first two add a little magic to the party, but also use armor, and weapons well, s they can get some use out of the caddy bag full of random loot you have them hauling around. The latter two are a Heavy Archer, and Immortal masher of a tank, I have builds for all of them, if you're interested.
Or, if you're really just that in love with Faendal (I fully understand, I wish he was marrigible) go back to your last autosave. I usually reserve that for irreplaceables like Eola (She dissappears if you dismiss her, and aren't married so she just goes Home) but I do have a soft spot for pretty little Faen...