Throughout the entire night, Fritz is functionally identical to Jeremy Fitzgerald in every way. Strangely, he even sounds exactly the same as Jeremy when breathing with the Freddy Fazbear Head on. However, based on the fact that his notice of termination listed "odor" as one of the reasons for him being fired, it is possible that he was also sweating and/or releasing bodily fluids similar to Mike, suggesting a panicked, nervous behavior. Another possibility is that he is another alias of Michael Afton, and the “odor” is just his rotting corpse.
After Jeremy completes Night 6, he is presumably moved over to day shift. Fritz takes over for Night 7, but is immediately fired the following day for tampering with the animatronics.
Similar to Mike from the first game, Fritz was fired for "tampering with the animatronics" and "odor" upon completing the Custom Night. However, unlike Mike, Fritz was not given "general unprofessionalism" as a reason for his termination.
Because of this, some people thought that Fritz is Michael Afton under the fake alias (as with Mike Schmidt), with him trying to fend off animatronics by frantically raising/lowering the animatronics' AI and his "odor" actually coming from Mike's rotting corpse.
On Fritz's pink slip, it reads that Fritz is the third employee, hinting that there was a first employee who took the night-shift job before Jeremy.
This is also hinted by Phone Guy saying that the guy who worked before Jeremy was complaining about "conditions", and how the animatronics attempted to enter the office. The first employee was then put on the day shift.
Fritz's breathing while wearing the Freddy Fazbear Head sounds exactly the same as Jeremy Fitzgerald's.
This may be due to game engine limitations only allowing for one breathing sound effect, or it may be an oversight by Scott Cawthon.
Fritz Smith is probably a combination of the two main guard's names: Mike Schmidt (Smith), and Jeremy Fitzgerald (Fritz).
Fritz Smith breathing with the Freddy Fazbear Head on.