So I've been Invited to play in an Only War campaign. It has been requested that I play a sanctioned psyker. I'm still new to the 40k universe and am not sure what the stereotypical psyker personality is. Any help and suggestions on a direction to go would be appreciated.

In one word? Unhinged .

Sanctioned Psykers go through a torturous sanctioning process that takes years, often scarring them mentally and physically, resulting in everything from the shakes to repeatedly broken and reset bones, or all hair - on your entire body - falling out.

Going through years, possibly decades, of training after possibly years aboard the black ships, which regularly drives people (well.. psykers) insane due to the soul-crushing despair and singular lack of actual soul within it's holds, psykers are heavily indoctrinated and many likely taught to hate themselves for the sin of being born with the psyker gene.

They also go through surgery, embedding psychic limiters deep in their brain, to prevent psychic surges that might overwhelm the psyker in question - a psyker, by the way, that - at least initially, once he has the risks explained to him - likely lives in mortal consant fear that his head might explode at any moment, opening a portal into a realm consisting of literal nightmares.

There are many different directions you could take a psyker, but most or all could or should somehow reflect this. Although you might have been born a noble, or the guy at the desk next to you during your "studies" in the Scholastia Psykana being born on a feral Death World, what you have in common is that you are apart from mankind, and almost universally feared and/or reviled.

The only real friends and allies you likely have is from within the Scholastia Psykana and the Adepts Astra Telepathica, if only because they are the only ones likely to not treat you like human trash. All the while, you are performing a function that the Imperium needs, whether it's Sanctioned Psykers on the battlefield or Astropaths responsible for all long-range communication in the Imperium. You are necessary, hated, reviled, taught to hate yourself and other psykers, powerless in your position, powerful in person, completely alone, yet part of a family of likely arrogant power-players that cannot be ignored.

Have fun.

You were raised to believe that mutants are corrupt, their bodies and minds twisted by the darkness of their wicked souls. The most vile and corrupt of all mutants are psykers, their minds gateways to the Warp, to hell itself. Later, you discovered that you were just such a being... a mutant and worse: a psyker.

Perhaps, as a good and dutiful citizen of the Imperial, you surrendered yourself to the authorities, perhaps you ran and tried to hide... but in the end you were taken, one of thousands of nascent psykers from your home world.

You were loaded onto one the Black Ships, tested not only for your psychic strength but for your mental fortitude, your ability to resist the siren call of your own corrupt soul. Of the thousands from your homeworld, and the thousands more from other worlds, most were found wanting, dying in the testing or killed by the guards before madness and corruption consumed them. You were not, your will was strong enough. You lived.

On Holy Terra, the survivors were taken and divided. Some, the weakest who had survived were lead away never to be seen again. The rest were taken to redeem the sin of their polluted birth by serving in the Astronomocion, burning out their souls to guide ships trough the Warp, or as Astro-telepaths, sending messages from world to world. You among the few chosen for another purpose, redeem your soul by using your powers to serve masters of the Imperial and defend humanity against the alien, the heretic and the mutant.

Your service offers you a chance for salvation but carries with it the ever present risk of damnation and death. With every use of your powers, the corruption of the Warp calls to you. The blessed humans around you hate and fear you as an abomination. You are a slave, in service to a master who despises you, who will use you and discard you and forget your name, but only through this enslavement can you save your soul... or so they have taught you.

Thanks for the descriptions. They are much appreciated, and give me a great idea on where to take this character

Just as it links to this topic...am I missing something somewhere? Psychic Discipline seems to be missing. Do Psykers in OW have free reign to pick from any discipline?

I believe so. The XP cost, added to the chain of prerequisites some powers have, and the ability to/cost of raising your PR is likely supposed to balance that out.

Yeah, no Psychic Disciplines in Only War, unfortunately.