To jump on a popular bandwagon, here's a brief rundown on Krycek and how he's being played here at Fandom High.
Original background post, 9/29/05 The canon background information still applies, the last bit? He's found some things out since then that take a little explaining.
Krycek at FH
Bear in mind that Krycek was introduced at the very start of the game, and I had no clear idea at the time of how Krycek would be in an 18 year old body but also have all of his adult memories. This was the plot that came about in an attempt to make it vaguely plausible (at least by X-Files standards, ha) and give him some motivations for me to work with.
So back before Christmas, Krycek found out that he's a clone, created by the alien rebels from his canon timeline with DNA they collected from his body when he was unceremoniously executed (season 8, ep 21, "Existence").
At one point in canon, Krycek went to Mulder and told him about the rebel faction of aliens, who were attempting to prevent the full-on invasion of another set of aliens and the inevitable enslavement of all humans on earth.
For my purposes, the rebels took this as a sign that he was an ally of theirs and, upon finding him dead, decided to attempt cloning Krycek in order to recreate a possibly helpful and important ally with ties to and information on many things that could help them out in their continuing efforts to battle the invading force.
Most of their clones that we see in the canon are alien/human hybrids, who physically resemble whomever the source DNA was taken from, but tend to act much more like the aliens and have some internal characteristics that set them apart (such as blood that emits a fatally toxic gas, yes, what fun). Their initial attempts at cloning produced a hybrid Krycek clone that didn't quite do what they wanted, or just sort of didn't fit the mold of "sociopathic assassin and all-around ethically challenged asshole" that they were looking for. There might be a few of them wandering around still, but they then attempted to make an entirely human clone, which is how this Krycek came to be.
Somehow because X-Files is great for the handwavey metaphysical crap, the essence of the original Krycek found its way to the new clone body and latched on. The aliens decided to plonk him down at Fandom High in hopes that social interaction would encourage him to be a little less sociopathic and wee bit more willing to help them out once he found out that they'd bestowed upon, given that Krycek's loyalty at the best of times is entirely up in the air.
And then Krycek went to find out what was going on, hating the idea that he was suddenly given new life with no explanation because he's a highly paranoid bastard like that, and discovered the rebel aliens' plans for him. He wasn't pleased, to put it lightly, and while he was there (with Jake Gavin Jr, who tagged along because he wasn't about to let his boyfriend hare off on a trip to his homeworld when there was the possibility of him never coming back) an alien shapeshifting assassin discovered his presence and pursued the both of them.
It ended with Jake getting shot in the head and Krycek killing the assassin with a spike to the back of the neck - the only way of killing the otherwise invulnerable sons of bitches. Jake recovered though, thankfully, even if the getting shot in the head was a little more traumatizing for Krycek to witness than for Jake to go through.
So, returning to Fandom High, Krycek decided to ignore the rebels and take his new lease on life to do whatever the hell he wanted. Currently, that includes graduating from high school (again, in his mind, even if the body's entirely new to the experience) and enjoying his so far disturbingly stable relationship with his boyfriend.
Relevant Links
The Plot, pt 1 The Plot, pt 2 The Plot, pt 3 Notes and Errata (or, how you might find a knife in your back on any given day)
Krycek's an asshole at heart. He doesn't take really great pains to hide it, but he does present a veneer of socially acceptable snark and jaded-ness or a very nice show of politeness and ass-kissing depending on the situation. He has few very good friends and doesn't really go out of his way to make more, but he's not averse to meeting people or getting acquainted to most any level.
That said, he is, as well, somewhat influenced by being physically and mentally younger - the memories are kind of layered on top of things, so he's not just an adult in a teenage body. He's uncertain sometimes, and is still learning how to be kind of "normal" in preparation for a life sans-Oiliens and various Consortia. He can be playful, have fun without being entirely bogged down by his past experiences, and is a little more liable to be drawn in by people's attempts to be friendly.
THE END
Questions, comments, sharks? You know what to do.