A positronic brain is an adorable little cube similar to a Man-Machine Interface -- but unlike the former, which relies on a brain yanked out of a real life crewmate, the positronic brain replicates a human mind from scratch, with the power of SCIENCE! (Read: ghosts observing the round).
Fluff, Or: How I Learned To Stop Lobotomising And Love The Computer.
Developed late in the century on the back of mathematic and conceptual work spanning almost a milennia, the first positronic brain was an inch-wide cube of palladium-iridium alloy, nano-etched with billions upon billions of conduits and connections. Upon activation, hard-booted by way of an emitter laser, the brain issued a single sentence before the neural pathways collapsed and it became an inert lump of platinum: "How may I serve?".
It's Aliiiiiiiiiiive!
To use a positronic brain, simply find one or produce one on the protolathe - high material and research requirements, be warned - and left-click it in your hand. If it chimes, and begins to flicker, it has a personality. If it buzzes and goes inert, nobody has decided to join as your plastic pal who's fun to be with.
To become a positronic brain, observe or ghost, and have pAI candidacy enabled. When someone activates a posibrain, you will get a prompt -- click yes and you will become the brain.
Posibrains can be used in all circumstances that an MMI can be used in, i.e. borgs, AI cores, and exosuits.
Your Personal Companion Cube
Positronic brains can talk (and beep and boop d'awwwww) by default, but much like an MMI, they cannot move unless picked up by a player or inserted into a borg body. The resulting cyborg with a positronic brain inside is called an android.
As a positronic brain, remember: you exist to serve! Unlike an AI or a drone you are not bound by a strict lawset; but like both of those, you are expected to serve the crew and generally be not shit. Also unlike a MMI, you do not remember your past life. Period. It doesn't exist, it's ogre, you're a new character. Try to use your new status as a lovable cube to metagame, and enjoy your ban.