#1 DO NOT KILL without express permission from mun. I am going to try to keep him from getting killed at all, or often, so if anyone thinks up a plot reason that he needs to die, I'd love to have a discussion about it to see if you can persuade me.
Maiming/injury - he can be harmed, since he is not a physical fighter and aside from some softball punches, probably wouldn't fight back much. He has no weapons skills whatsoever. I prefer no permanent disfigurement. A cool scar is okay. Permanently altering his mental state is a no, but putting him through some small tortures that he can heal from is fine.
Mind-reading/empathy - anyone who tries to look into his mind will find it full of three things: pop culture, his past, and languages. He spent his formative teenage years in L.A., so he is fairly well indoctrinated in American pop culture. His past consumes much of his mind, though not consciously as much these days. The languages come from his power. He has no way to defend himself against intrusions or mind-control, but he did require a stronger control over his faculties in order to wield the Oricalchos without being consumed by it. He is wary of being duped, used, or controlled (because of Dartz) so the minute he realizes that's what going on, he will back off and even flee. It is possible that the Oricalchos has left a bit of a stain on his soul, certain empaths or magic-users might be able to detect it.
Hacking skill - he does have great skill with computers, canonically. He hacked into one corporate mainframe (Industrial Illusions) and then used it to hack into another corporate mainframe (KaibaCorp), for the purpose of transferring majority share of the stock from one company to the other. He managed to do it without the genius CEO of the company noticing until it was too late, so he's pretty good. Really good. He will be hacking encryptions as soon as he gets the hang of the technology, though he may not necessarily use it against anyone. He may just be a creeper and read so he knows what's going on.
Affiliations - while Alastair regrets his past (see below), he is not a suddenly changed man. He has been using underhanded, illegal means for so long, it's his way of life. He wants to try, though, because he knows deep down that his little brother would be ashamed of him if he knew that Alastair was a criminal. He is extremely malleable and could go either good or corrupt, but he will not be subservient to a villain mastermind again. He is wary about being manipulated. He will want to know what's in it for him, and the minute he senses betrayal, he will turn away.
Alastair's first power is over languages. At the start, he could immediately understand and respond in any language he hears, but a few minutes after hearing it last/using it last, it fades and he doesn't remember exactly what was said. So, he can only translate if he's right there at the moment, reading or hearing the language. Over time, it will develop into a command of all languages regardless. This includes languages he can't reproduce with a human tongue, like beast languages (Godzilla, et al) and Cybertronian. The only languages he speaks from his world are Croatian (his mother tongue), English, and Japanese.
Alastair's past, for those not familiar with Yu-Gi-Oh, is that of a henchman. He worked as one of three inner-circle henchmen for a sorcerer called Dartz, who claimed to be 10,000 years old and Atlantean in origin. In Dartz's day, a meteorite stone called the Oricalchos "revealed" to him the darkness of the human heart and possessed him, giving him a firm belief that the human race had to be exterminated in order to set up a new world where peace would reign. This caused the downfall of Atlantis, among other things. The means of destruction was to be a great beast called the Leviathan, which Dartz controlled. But the Leviathan was defeated and sent into dormancy for all those years, while Dartz roamed the world collecting souls of the living to resurrect it. He plucked Alastair out of a refugee camp when he was around 10 years old, after his mother and brother were killed, and groomed him to be one of his top soul-takers, indoctrinating him with the same belief that the world should be destroyed and all humans wiped out to create the new order. He also got Alastair to believe that since KaibaCorp was responsible for the weapons that killed his family, he should exact his revenge on the head of KaibaCorp. This was Alastair's aim, at the time this plotline occurred in the series, but he was eventually defeated and lost his own soul as a penalty. Dartz very nearly resurrected the Leviathan, but in the end was defeated and Alastair's soul returned to him. He fled from the hospital where Kaiba took his soulless body, and escaped to Europe. THIS IS ALL ACTUAL CANON, NO HEAD-CANON INVOLVED.
Specifically, in his brother's case, Alastair was attempting to hoist him up into a tank belonging to their own side, so they could evacuate the camp when it came under attack. Enemy helicopters came over the hill and blew up the tank, injuring Alastair and killing Michael right in front of his eyes.
What is important, though, and what Alastair did not find out in canon (but has since learned through Kaiba and through seeing his own subconscious memories thanks to Soldier Blue), is that Dartz was actually responsible for killing Alastair's family in order to turn him to the dark side. He charaded as Gozaburo Kaiba to fake "evidence" that KaibaCorp supplied weapons to the enemy army, and masterminded the deaths of Alastair's mother and brother in order to make him malleable and open to being approached and recruited. He did the same to Alastair's best friends/fellow henchmen, Raphael and Valon. Dartz admitted it (even bragged about it) to Kaiba, who has in turn told Alastair in the City. Now that he knows this, Alastair is even more wary to avoid situations that remind him of his past.
SO...tl;dr upshot of this is, anyone wishing to look into his past or find a topic to discuss with him, the keywords are: war orphan, soul-stealing, sorcery, Dartz, Oricalchos, Atlantis, dueling, and his little brother Michael. His emotions regarding his past are going to be filled with regret and grief. His little brother meant everything to him, and losing him scarred him for life. He carries his brother's charred action figure of a mecha bot everywhere he goes.
The Oricalchos was a power that rooted itself in the wielder's soul, via a shard of the meteorite which they wore somewhere on their person. Alastair wore his on a necklace (see icon). It was fueled by the person's dark side - their hatred, anger, resentment, and viciousness. Only people with a nearly-pure heart, or those who genuinely repented and felt remorse for their actions, could resist the Oricalchos. Those who used it willingly, like Alastair, could call upon it to take souls, basically, but also could use it to do a few other tricks. Since everything in YGO is rather ritualized, he could only take souls during a ritual duel, in which he used a card inscribed with the Seal of Oricalchos, which essentially turned the duel into a real magic battle with real monsters, at the end of which the loser forfeited their soul. But, canonically, Alastair also used his power to induce hallucinations in Seto Kaiba, so he could show Kaiba his own childhood in vivid reality, and when he lost his senses out of rage and grief, he blasted out all the windows in a jumbo jet, damaged the engines, and sent it plummeting toward a mountain peak. It affected him long-term. He remembers all of the things he did, and he isn't proud of them anymore.
UPDATE, 1/10: as of 12/27, Alastair went on a trip into his subconscious memories thanks to Blue, so he has recovered his soul-memory of being inside the Leviathan, participating spiritually in the monster's destruction, and having a moment's reunion with his brother's soul before Michael went to the afterlife and Alastair regained consciousness. Thus, his connection to Kaiba is stronger, and his traumatic memories dealing with Michael's death and his own participation in nearly dying are closer to the surface. Strong mind-readers will be able to see what Blue unearthed; that is, the memory of his soul being almost absorbed by the Leviathan, and choosing at the last second to aid the pharaoh in rescuing them rather than die.
UPDATE, 3/11: Alastair was ported out in November 2010 for a power update. He now has the power of the Oricalchos back, in some altered sense from what it used to be. It was awakened in early March after Bakura's death and he will now be dealing with the trauma of having it back and figuring out what that means. There is a separate post in this journal with the information on how the Oricalchos power works.
Questions and clarifications? Leave me a comment!