faendal was helping me in my quests. in one of those, while i was walking by the the way, he was too slow and i left him behind. i thought the "fast travel" would bring him back to where i was. well... i didn't. now i walk alone. i tried to hire other people, but they say i already have someone. i even went to his house, trying to reach him, but there was nobody there. please, someone can help me?

if you are pc you can use the console to teleport to him or port to wherever he is. use the wiki to find his id code and the console command.

if not on pc, fast travel usually does it. since it hasn't, odds are he's gotten 'stuck' in the landscape somewhere. good luck. that handy dandy prior save you have could probably help you out.

On my game Faendal just wanders across Skyrim now. I got a job from the companions asking me to beat him up, maybe that's why?

Check the dragonreach dungeon in whiterun.

He's set to guard faction for some reason, sometimes he ends up in one of the prison cells.

Yeah, he pops up randomly all over the map after being a follower for a while. My battlemage and Jordis ran across him outside of Helgen when we were atacked by a dragon. I don't think he helped out even though he was RIGHT THERE. Later, with J'zargo in tow, he spotted Faendal near Windhelm. Not sure where he could be, if Fast Travel hasn't worked.