Who is Klaus in the books?

In an interview with Entertainment Weekly, Julie Plec had this to say about Rose dropping Klaus’ name in the final moments of episode 208:
The mystery of Klaus and when we’ll see Klaus is going to be the same প্রশ্ন of ‘When will we see Katherine?’ from last season. We’re gonna learn a lot about him and be hopefully very afraid of him before we ever lay eyes on him. That’s gonna be a nice, big, long healthy tease. It’s so funny, the way we dropped the bomb of his name last episode [Rose ব্যক্ত The Originals, the oldest vampires, will be coming for Elena, and they'll be doing it for him, Klaus], Kevin and I were laughing. We’re like, the book অনুরাগী are gonna lose their freakin’ minds in this moment, and the rest of America is gonna go, ‘Huh… Alright. Who’s that?’ [Laughs] But it’s one of those moments that you’re like, I don’t care because I know the million book অনুরাগী are gonna be so excited that it’s worth it.”

And she was right! In the Vampire Diaries book series, Klaus is actually Katherine’s maker and, as is par for the course with the TV series, they’ve switched it up to keep book অনুরাগী guessing. In the show, Katherine essentially makes herself, after tricking Rose into giving her blood; her history with Klaus is now much, much different. In the books, Klaus is the Big Bad of book 4, Dark Reunion, come to wreak merry havoc on Fell’s Church, Virginia (because he was apparently quite bored). Tall, Nordic-looking, powerful, charismatic, and creepy as hell, the Klaus of the বই claims to be carrying a bronze ax in his earliest memories. He makes passing references to the fall of Rome, fighting with Alexander’s army, and being in the Trojan War, so আপনি know he’s old. He also brags a lot, but what makes him scary is that his follow-through on ব্যক্ত boasts is excellent. Also, he’s so insane he’s wrapped back ’round to sane again, if আপনি can imagine that. (L.J. Smith once stated that she based the character on Rutger Hauer in The Hitcher, possibly one of the greatest horror চলচ্চিত্র of the eighties.) He also has a penchant for quoting কবিতা (Kipling, Auden, Housman, and Roethke) and has a soft spot for fifties সঙ্গীত (see if আপনি can ever listen to The Spaniels’ “Goodnight, Sweetheart, Goodnight” the same way again after পাঠ করা Dark Reunion).