"The janitor" is a character who does not appear in Mirror’s Edge. But we will talk about him anyway!
He is a mysterious character who left several mysterious elements throughout the game.

His office

In chapter "Ropeburn", at some point you arrive in a maintenance area and you have to climb up to reach a button in the janitor’s "office". In this office (a bit of an overstatement) there are several items we will look into.

** Scruffy

Scruffy is the janitor’s rat, it’s in a cage on the ground, you can’t miss it. You will find more info about this little fella on the page dedicated to rats in Mirror’s Edge.

** The white board

On a wall, you will see a white board with a note and a drawing:

Both have a very childish style and there is a big typo in the word "remember". The drawing shows that the janitor is wondering who is the hulk-like person who keeps destroying all the red doors of the city.

** Janitor of the month diploma

Next to the white board is a "Janitor of the month diploma". Once again, this page has been entirely hand-written (and drawn) by the janitor himself.

There is also a typo on this page, in the word "presentid" and again, it looks very childish in terms of writing/drawing but also in the mere fact of awarding a diploma to himself.
This diploma thing might be linked to something else: in chapter "Pirandello/Kruger", after a hard fall in a yellow stairwell, you get in an orange room full of SWATs shooting at you. Before escaping, look on the right (instead of taking the door on the left): there is a desk with a computer. You will see a note on the screen and an actual "Employee of the month" diploma on the wall.

So it looks like Milton .T Heft is the janitor’s brother and he has received a legitimate employee of the month diploma from his boss Owen. The janitor was probably jealous or disappointed of not getting such a reward so he made one up for himself. He then wanted to call his brother to tell him the good news.
We still don’t know much about the janitor but he seems to be simple-minded (even Milton’s boss says so) and have childish reactions in general. On the contrary, his brother seems to have a successful career at Pirandello/Kruger.

The drawings

When finding runner bags you have probably seen graffitis and/or drawings next to it. The writings/graffitis are from Runners, but the drawings really look like they have been made by the janitor. So he might have found Runner’s stashes, which raises more questions about his real identity. Moreover, even if his drawings have a childish look, they appear to hold a deeper meaning, more symbolic.
But we will analyze this further in a page dedicated to these messages that always come along runner bags.