Right so arguable the most important and fun part of this sections, your giant robot! It's awesome and highly customizable, it kills things dead in various ways!. And most importantly chicks dig them machines along with dudes and well pretty much everyone including other pilots. But what are they? What are their functions? And most importantly where did the come from, how did you get one?
Well for starters your mechs are highly customizable machines of unique composition that came from the pre-cataclysm era. Too be honest not too much information is known even by the techies (curse those corrupted memory banks and broken computers!). There are also some lesser type of mecha called Walkers but they're a bit more cumbersome since they lack the supertech and are either derived from existing copies found as well as the occasional rare fancy types uncovered non-hunter affiliated people and partially reverse engineered from.
Suffice to say they were as previously mentioned machines manufactured by

and SCIENCE

! They are part mechanical and biological which means they got special features such as self repair and regenerating ammo and other nifty stuff. They also have a mysterious connection to the monsters in that if the biological parts are in contact with monster parts which is due to

and of course more science!
And that's were the customization comes in since you're not only just limited to finding spare parts and scrap but you can also hunt down the deadly critters and slap on part and get nifty stuff. See that giant floating electric centipede? Kill it and slap on the bioelectrical generators and yourself a new battery source, weapon, or shield! There is one caveat and that is the fact that not all of you might be technically inclined or have the tools which is why you have the handy stations scattered around.