It's not nearly as fleshed out, no, but as you learn about AI and synthetic/organic conflicts and the scale/scope of the Reaper cycle throughout the series you can still make a lot of very logical deductions about the Catalyst when you meet it from the very base ME3 game.Dude, there's zero of that in the vanilla ME3 game. Not even a slight hint. Nothing, nada. The catalyst kid came out of the left field and gave me three friggin' buttons to press!!! Game over! It's like an ending to a completely different game that I didn't play and by some strange error was put in ME3 at the very end.
You know what? I don't want to go back to this - fuck Bioware for their lies and huge slap in my face for the years I've spent in Mass Effect 1 and 2. And let's leave it at that, OK?
It's evident that it is a synthetic (AI), it is older than an organic can imagine, the Catalyst has been around long enough to conclude that synthetic/organic coexistence is not possible, the Reapers have been following the Catalyst's instructions for much of that time, the Reapers are not necessarily the Catalyst's only solution and the Reapers do not simply 'kill everything' but only harvest those advanced enough to be at risk of producing a 'dangerous' synthetic. It doesn't tell the full story but certainly provides a framework for what the Catalyst is and why it exists.