I don't actually know much about this game, (I haven't even found the lost rat yet) so feel free to comment so I can fix incorrect things or add things!
This guide is to help people overcome the fear of the unknown and help to evade the hunter.

The thing hunting you throughout the sewers doesn't have an actual name as far as I know, but I'll just call it the Hunter for the guide.
Due to the fact that the game is darker in screenshots, I won't use much, but I'll brighten them when i do.

This is what it looks like:

The Hunter is quite big when seen in-game, about 4x the size of you.

Generally, the best idea when you see the Hunter is to do a 180° and book it the other way. Be careful when running so you don't get caught on the ground, or run into a dead end. Sometimes you can outmanuver it by wall-jumping, or being speedy though.


The Hunter itself seems to be a little confused about catching you, but won't hold back about trying to find you.


Staying in the same area for too long, while being close enough to the ground, will inevitably bring it to you, so it's smart to stay moving and go another way when it's in front of you.
(Gray circle is The Hunter peeking around a corner while I'm on a pipe)


Pipes are a safe haven, but since it's a good idea to stay mobile, and it's sometimes time consuming to get onto the pipe, it's not that good of an idea except when it's behind and you can't run much.
Adding onto this is the fact that it'll stay in a perimeter around you when you're on one.

The speed of the Hunter isn't that fast, but you can't see him coming most of the time.
Turning corners are probably the most dangerous thing to do in this game.
(Higher difficulties will make it faster, be wary!)

There's some walkways with railings in the halls, and it can't go through or above the railings, it has to go around to get to you. It's normally a good thing to stay in the walkway so you can switch between the "road" and the walkway and run when you see it.