ReactOS Portable Systems Group
A bunch of people very knowledgeable in ARM programming, also known as the ARM ninjas (AKA Code Ninjas).
Trained in the secret arts of ARM system programming in the depths of the internet, they are masters of the craft. From the shadows, they commit code that actually works (most of the time) and dare to approach the untouchable (the ROS memory manager). Their true identities are closely guarded secrets that only the highest echelons (Fireball) are aware of. And no, Alex isn't one of them. We're (almost) positive of that.
They are known of sharp, witty commit messages, often depicting various ReactOS code failures in a quite sarcastic way. Note that even they managed to get ROS a bunch of regressions, luckily fixed quite fast.
There are rumors about a great power leaning behind ARM ninjas. Some say they are from Apple Kernel Team, where Alex Ionescu has passed his recent internship. Others point to IBM or Google.
Similarities are also detected between theirs and Alex's commit style, but all allegations in this direction are being rejected.