*late reply/answer* I can give you more detail about most characters, so this will be a long message. I know all the characteristics for most of the 2P!Hetalia characters.
2P!England (my favourite :3) - He wears a pink sweatervest~! He can cook VERY WELL and he especially likes sweets. He feigns innocence. Being asked to join him for tea is like being asked, "Do you want me to kill you?" His hair is a bit paler and his eyes are a an odd colour that I don't know how to describe. He gets along very well with 2P!America. Oh, and he isn't a gentleman.
2P!America (Very close to being first favourite with 2P!England for me~) - He's a douchebag! 8D His bomber jacket has a nuke on the shoulder instead of an airplane. He keeps a bat with him all the time. The bat has jagged nails that stick out of the end. He's pretty damn sadistic and it would be in any person's best interest if he didn't take a liking to them. He gets along well with 2P!Russia and 2P!England. He's pretty smart and doesn't eat all the time.
2P!Russia (3rd favourite for me! >w<) - To put it bluntly, he's Russia from the Soviet era minus the innocence. He's also a stick in the mud and he never smiles.
2P!Greece - Not lazy~! And he still loves cats. He has aristocratic clothing and he enjoys writing. He's got more emotions than 1P!Greece and he isn't really sadistic or insane or anything, but he is kind of rude. He doesn't really get along with anyone. Except his cats.
2P!Germany - Nazi and a sadistic asshole! 8D (That's really how Himaruya put it.) He likes BDSM a LOT. Way more than 1P!Germany does. He only gets along with 2P!Prussia and 2P!Italy. He's isn't very tidy, either.
2P!Prussia - Even more of a Nazi and sadistic asshole than 2P!Germany. He also likes BDSM way more than him. He can't get drunk no matter how much he drinks. He gets along well with 2P!England. As in whenever 2P!Prussia is depressed and wants to kill himself, 2P!England will come along and be like, "Wait! I'll help you (kill yourself)!"
2P!Italy - Not innocent. Can't cook. Nazi. Enjoys BDSM. NOT innocent. Keeps a riding crop with himself all the time. Not innocent. Did I mention not innocent? He's more of a pervert. o3o
2P!Japan - Sadistic, mysterious, socialises more than 1P!Japan. He likes to torture his victims to death. He smiles... as much as 1P!Russia does. He feigns innocence IF he feels like it.
2P!Canada - He is noticed. Very well by everyone. He's a lot like 2P!America, except he's more of a pervert. He carries around a hockey stick with him most of the time. He doesn't feign innocence at all, and he has either a smile like 1P!Russia or a dark smirk most of the time.
These guys have been released officially by Himaruya along with Nyotalia characters and their 2P counterparts (of which they are similar to the Hetalia 2P characters, just female). These are all of the characters that have been released so far. Their purpose is for more entertainment of the fandom and the DS Hetalia game that is coming out this year~ (I think it's for 3DS or somthing.) I hope this helps! ouo
If those are the only characters who have officially been released, I wonder who some of the others are... Like 2p France and Romano. Or what about these demon versions of the 2p America and Italy you see flying around the interwebs... Oh well...
Well from what I know the creator of Hetalia, Himaruya, was requested to design/recolor some of the Hetalia characters. 2P stands for 2nd player, as the recolors reference that in a video game (like Super Smash Bros) that if two people pick the same character, the seconds player character will be a different color. Hima recolored the Axis but only did the recolors of the Nyotalia!Allies. So fans of Hetalia interpreted them as the darker forms/clones of the original Hetalia characters. Many people making their own designs of the Allies and other characters Hima didn't recolor. The 2P's are more inclined to violence and are in certain ways opposite of their 1P Hetalia versions.
Hope that helped! If you have any more questions just ask!