This is probably going to end up tl;dr but feel free to pick out any info you want.
Notes for the psychics: I don't mind any psychics mentioning this around Kratos, though it might not provoke a very good reaction from him.
Although he doesn't look it, Kratos is over four thousand years old and so has a lot of past history. According to Tales of Fandom 2 he used to be Captain of the Knighthood of Tethe'alla, a country that was at war with Sylvarant at that time. He later left his high rank and the privileges of being part of the noble class of Meltokio to help Mithos and his sister Martel stop the oppression of half-elves. They later enlisted the help of Yuan who fought against Kratos on the Sylvarant side, and despite initially being rivals they actually became good friends.
Adopting him as an apprentice, Kratos was also close to Mithos. After Martel was murdered and Mithos became overwhelmed with grief, Kratos knew he had to stop his ideal of an 'Age of Lifeless Beings' but took no action, deciding to help Mithos instead. He now harbours a lot of guilt about his inability to face up to things and make amends for what he believes to be his own mistakes - for not being able to help Mithos move on from his sister's death and for not being able to stop him when he knew it was too late to do so.
Further on in his life he fell in love with a human woman, Anna, who changed his beliefs about Mithos' ideals. He helped her escape from the Asgard Human Ranch and they lived a life on the run being relentlessly pursued by Mithos' organisation, Cruxis. Soon they got married and had a son, Lloyd. Unfortunately, the Desian Grand Cardinal, Kvar and his men eventually managed to track Kratos and Anna down. Kvar removed Anna's Exsphere causing her to transform into a mindless monster. The dog-like Protozoan, Noishe, protected Lloyd from the monster at first, and Anna momentarily regained consciousness. She begged Kratos to kill her, but he couldn't bring himself to. Losing her mind again, Anna attempted to kill Lloyd a second time, and in a desperate attempt to protect his son, Kratos killed his wife. Whilst fighting off Kvar's men, Anna and Lloyd both fell from the cliff they were on. Kratos descended the cliff and found nothing but half-eaten Desian corpses. Now believing they were dead, Kratos lost all purpose in his life and went back to Mithos. He eventually found Lloyd again, although he still ended up betraying him. He also is shown to have suicidal tendencies, as he will essentially live forever or at least for an exceptionally long time. He already had no qualms about releasing the mana in his body to break Origin's seal and allow Lloyd to continue and stop Mithos.
What's okay to mention around him: Seriously, you can mention anything you want, but different things will obviously result in different reactions from him. Any mention of past sins, or love, or maybe even parenting in certain respects is likely to make him slightly depressed. He carries a lot of guilt and tends to hide his pain inside. He's also used to running away from his problems although he's trying to confront them more now. You'll likely have to poke him a bit to get him to talk, if you want him to. XD Friendship has also become a bit of a painful subject for him since Yuan has decided to end their friendship.
Abilities: Kratos is an angel and therefore comes with many abilities because of it. He has wings, although he hides them most of the time (even though Jacky wishes he didn't). He also has enhanced strength, sight and hearing. Having over four thousand years of experience, Kratos is a superb swordsman and he is able to use magic - both offensive elemental spells and curative spells (although these are limited).
Can I shapeshift/bodyswap/slash/etc?: I'm open to anything really but I'd prefer it if you discussed it with me first~ :3 I may or may not agree depending on the situation and characters involved. Although I don't mind slash, I'm kinda happy with Kratos' relationship with Jacky at the moment and I don't think he would be interested right now. XD;
Maim/Murder/Death: Again, I'd like to discuss anything before making a decision. I don't mind death as long as it's temporary. I'm not sure about maiming though. XD Depends on the exact situation.
Kissing/Hugging/Badtouch: Feel free to try it, but I doubt he would like it~ (Unless it's Jacky... >.>)
Easiest way to get on his good side: Respecting his privacy is always a good thing, although if he feels he can relate to you he might open up a little. :3 Uhh... he also appreciates non-judgmental people, though it always surprises him when someone forgives his past actions so easily. Also, generally if you agree with him, he'll like you moar. XD
Easiest way to get on his bad side: Easiest way? Do anything negative against Lloyd, and now Jacky and you'll be sorry. (He does care about Yuan too and still tries to defend him even if he doesn't know/care about it XD) He doesn't like people speaking ill of the dead either, especially if it's personal.
Favorite person: Lloyd is definitely the most important to him. Jacky of course~ And Yuan when he's not being a jerk. XD
Favorite person met on the comm: Of course, Jacky~ He's also beginning to warm to Shuda more, recently. And Ten as well actually, since he feels like he can kind of relate with some aspects.
Prefers: Women. X3