Atticus Rosenbloom is a character in the 39 Clues who may only be 11 years old, but he has an IQ of over 200. His only friends are Jake, his half brother, Dan, his friend he met online, and Amy, Dan's sister. He and Dan are friends partly because they share one major common: they both lost their mother at a young age. The other reasons were because they share a common weirdness and both have a fascination with ninjas. Right before Atticus' mother died, her last words were "Stay friends with Dan Cahill", up until recently, he thought she wanted him to keep his only friend after her death. She also gave him the link to the chat room in which he meets Dan. She suggested this because she knew Dan's grandmother - Grace Cahill - because Grace had told her the truth about the Guardians, Madrigals and Vespers.
He was originally a Vesper suspect, because according to his card, he has come from a family with a dark past, and in Mission 1: The Medusa Plot, he is one of the suspects included in the list of possible suspects for Vesper Six, along with his brother, Jake. However, in 'A King's Ransom,' he is revealed to be a Guardian. The Guardians are yet another family that knows about Cahills and the 39 Clues. He is also kidnapped by the Vespers and held hostage in this book. In the first chapter of The Dead of Night he has some Nanotech that he got from his mother Astrid's tech guy, Max Beezer. Beezer left most of it to Astrid's assistant, Dave Speminer, but left some cool ones to Atticus. He may be a Madrigal by joining.
Atticus is first mentioned by Amy, when she explains that Dan became a loner after the Clue Hunt and that his only friend is Atticus. Later in the book, Dan texts him asking for help with figuring out the writing in the back of the real Medusa. Atticus calls him and informs him that the place is Porta Sanavivaria, in Colloseum. The Cahill siblings go to Rome and meet Atticus and his brother, Jake (who is taking lessons there and Atticus is with him), in the Colloseum and together they find the Il Millione. When realising that Dan and Amy are not willing to give the manuscript into a museum, he is very sad, feeling betrayed by his only friend. Jake turns the siblings into Interpol and later Atticus leaves some messages in Dan's cell phone, informing him what Jake did and that they should give back the Il Millione.
Atticus is trying to get over his hurt and shock, getting back to his routine in Rome. When Katja Mavel calls about what seeems like Amy and Dan and refers to the Neuscheanstein castle and Jake says, ironically, that they might steal the Mad King's body, Atticus stops Jake from calling Interpol about the Cahills, because the night his mother, Astrid, had died, she had told him to "Follow the sparrow to the Mad King's castle". She also told him that she passes all Guardianship to him, refering that it's up to Atticus to guard the place. Atticus had believed that she was hallucinating but now he wants to know about the Mad King, as he learns that he's real. So, the Rosenbloom siblings go to Prague, where thet meet Amy and Dan just when they're about to enter the library. When Jakes opens Amy's backpack, a photo of Jane Sperling falls in the pavement on Atticus's feet and he says that Jane Sperling is his great-grandmother. The Cahill and the Rosenbloom siblings exchange info and decide to go into the library, where they find the de Virga. Later, during the drop, Atticus gets kidnapped.
Atticus is with Casper and Cheyenne Wyoming and they're ready to take off with their jet. The Wyomings then stop to see if the getaway jet is ready. Then Atticus complains that he is getting sick from the smell of sticky buns and motor oil. Before, he remembered that his mom's tech guy, Max Beezer, gave some of his tech to him and his assistant Dave Speminer. He asks for his key chain and Cheyenne finds that on the key-chain that there was a transmitter (flashdrive) but she destroyed it. But the hand sanitizer was really a hidden transmitter. Atticus types a coded message to Dan and Amy via the transmitter in a men's bathroom. The Wyoming twins take Atticus to a Vesper stronghold in Goreme, Turkey. There, they force Atticus to decipher a code. Atticus activates the self-destruct system, and he runs away before the place explodes. Outside, he is finally reunited with Dan, Amy, and Jake.While Amy and Dan are taken into Interpol and Luna Amato gets them out and the siblings flee, Atticus with his brother still wait in the hotel. Then, Amy and Dan get back and the four of them fly to Samarkand.Atticus did the research while in the plane and he is telling the others. While in the tour with Salim Umarov, Atticus thinks he saw Casper, but he was just a blonde tourist. Later, when Amy and Jake go to Estelle Urb, Atticus and Dan gothe street to buy snacks and a man scares them when he relays a message to Atticus from his father, telling him to come home. Atticus, with his brother and Amy and Dan help find the astrolabe and the three of them make the drop, while Jake is under arrest.
Atticus is exceptionaly clever, his IQ being 200-plus and he is mature for his age, according to Dan. He doesn't have any friend except Dan, because he doesn't have much things in common with normal kids and they think he's weird. In actuality, he became friends with Dan at first because they had both Atticus likes History and learning languages, but he also likes playing video games and practicing ninja-moves. He uses the phrase "in actuality" very often and Dan has stated that he makes History events sound interesting.
Atticus is African-American and has dark hair and brown eyes. He wears round, red glasses as shown in the cards. He usually wears plaid shirts and beat-up Vans, as he described himself in the beginning of The Dead of Night. Atticus is a little sad with his physical appearance, feeling that it's hard being a "skinny nerd with glasses" as he thinks of himself with a brother and a father that are very good-looking.
- Atticus is extremely intelligent.
- He is skilled in researching, like Amy.
- His Latin is better than Jake's.
- He knows Spanish.
- He has finished college in his spare time.
- His keychain contains seven keys, five plastic store reward cards, a screwdriver, a flashdrive (a disguised transmitter, destroyed by Cheyenne Wyoming), and a tiny but festive looking can of 'Germ Away' (actually a disguised device, use was currently unknown until in The Dead of Night it is revealed to be a liquid transmitter).
- He is the first person to be captured by the Vespers and then escaped, before Phoenix did so in Shatterproof and the others in Day of Doom.
- Astrid's tech guy, Beezer, might have been a Cahill or Guardian due to him giving Atticus spy gear.
- In Day of Doom, Atticus uses an anagram app to find out Dave Speminer is an anagram of Damien Vesper instead of using his brain.