'"You can't call a ninja lord dweeb. You have disgraced the family. You must commit seppuku."
Dan Cahill was born to Hope Cahill and Arthur Trent, along with Amy. In 2001, his parents died in a fire created by Isabel Kabra. The fire made Amy and Dan have only two main relatives: Aunt Beatrice, and Grace Cahill. He and his sister went to live with Aunt Beatrice. Beatrice hates the children and they live with a series of au pairs in a separate apartment. Dan and Amy blame Grace for not letting them live with her. They often visit Grace on the weekends.
When his grandmother, Grace Cahill, dies, Dan Cahill finds out he is part of the most powerful family on Earth, the Cahills. He sells his baseball cards and collections for about $3,000.00 to help fund the Clue Hunt. Dan is frequently called "dweeb" and "idiot" by Amy Cahill in this book, The Maze of Bones.
He and Amy are to find out the next clue in Book 2 by solving the song sheet of W.A.M. (Wolfgang Amadeus Mozart). Thanks to his photographic memory, he could re-create the lead they had early in the book that was destroyed by Saladin accidentally though they are tailed by their competitors.
The relationship between Amy Cahill and Ian Kabra greatly increases in Book 3. When Ian and his sister, Natalie, join an alliance with Amy and Dan Cahill, Ian develops a crush on Amy while trying to trick her into falling in love with him. However, the relationship ends in disaster when the Kabras betray the Cahills leaving them to die in a cave but Ian's crush on Amy grows more than ever.
They are having doubts if Grace was trying to help them in Beyond the Grave, Book 4. But in the end, they found out that she was really helping them and in the end they found the clue with the help of their adopted cat Saladin.
Next, in Book 5, they went to Russia searching for the Lucian clue (Amber) involving the Black Circle. Nataliya the Lucian helped them but Irina Spasky caught them in the Black Circle and the man in black(actually Fiske Cahill) saved them.
They went to Australia in Book 6 and finally meet the cousin of their father, Shep Trent. They found that Shep Trent looks a lot like their father so much. Though there time to spend time with Shep was short because Isabel Kabra was on their track. Though because of this, Irina Spasky from the competition sacrificed her life for Amy and Dan which shocked both Dan and Amy who thought Irina was evil.
In Book 7, The Viper's Nest, Dan and Amy travel to South Africa. There, he discovers his parents were Madrigals. He arrives at the conclusion that he and his sister Amy are Madrigals but acts like it doesn't bother him.
Dan and his sister Amy travel to China in Book 8, and after complications, Amy Cahill confesses that she thinks that their parents were bad people. Dan is crestfallen, and storms away from Amy, shouting that he hates her. Amy is left in the middle of Tiananmen Square. Dan is cornered and kidnapped by the Kabras, and Dan is suffocated until he turns unconscious with a handkerchief filled with chloroform, a knockout chemical. When Dan finally awakes, he is in a Chinese factory that manufactures Jonah Wizard lollipops. Jonah appears after Dan breaks an appliance that was about to kill him, and then whisks him away with him into all of his concerts, pretending to be his friend. After breaking into the Terracotta Warriors, stopping a crowd crush, etc., Dan explodes with anger and yells at the whole Wizard family that he is a Madrigal when they try to trick him into thinking he and his sister are Janus branch members. He then travels on a train secretly to the border between China and Thailand, where he re-unites with Amy and Nellie. They travel up Mount Everest aboard the only A-Star helicopter in the world, and have a desperate fight at the peak with their cousins, where Amy rescues Ian Kabra instead of saving the Janus Serum. The book ends with Amy and Dan loving each other again, and they find the next Clue, Silk. The trio heads to Jamaica, where the book comes to a finish with both Dan and Amy questioning Nellie's loyalty and character.
In the middle of the ninth book, Dan meets a man named Lester Dixon. Then, he entrusts him with the box, which they think hold the next clue (which LEADS to the next clue) but the Kabras want it. They send two bodyguards after Lester, and both of them find their way in quicksand. The two bodyguards manage their way out with Lester's helpful instructions, but accidentally pulls Lester in and he dies from the incoming tide. This causes Dan to be in shock, putting him in the hospital. Then, Dan and his sister Amy are granted active Madrigal status in this book and become they youngest active Madrigals. Nellie is granted active Madrigal status also and becomes the first Madrigal to have active Madrigal status without Cahill blood.
At the end of Book 10, Dan, Amy, Hamilton, Ian, Natalie, Alistair, Sinead, and Jonah find their way into the Gauntlet, which is a Madrigal stronghold. But Isabel Kabra is already there but after a fight, the other competition can't trust themselves with the clue and hands them all to Dan and Amy. They are also adopted byFiske and Nellie although Nellie acts like their big sister.
It was seen in the Black Book that on his first day back to school he got sent to the principals office 3 times. Also Amy and Dan went to Grace's bank in Switzerland and it is said by Amy Cahill to Jonah Wizard that the Vespers are coming.
It was seen in the last part of Vespers Rising that Amy is the new guardian of Gideon Cahill's ring. While they are in Switzerland, Dan notices that the guard doesn't seem to fit in, unlike Amy and Fiske. He shouts a warning just before Casper Wyoming tries to kill them. When they escape, they go to the mountains. While trying to escape Wyoming again, they are dangling by a single cable from a ski lift. As Dan, Amy, and the other passengers are dangling, Dan wonders if he took the serum, would things be different. He could, with the strength of the Tomas, rip open the roof and battle Wyoming. Amy sees this and becomes worried. She knows that they must learn to be strong without the Serum. She swears to herself that next time the Vespers come calling, she and Dan will be ready for them, without the Serum.
Dan is rather annoying to Amy because he is childish, but he cares deeply for his sister inside because she is the only loving family he has left. Dan loves to eat junk food and is obsessed with ninjas, foreign snacks, burp and fart jokes, and anything to do with video games. Dan Cahill and Amy get into plenty of fights and arguments, but they always forgive each other afterwards because they only have each other in the world. Even though Amy calls him "dweeb," for her that is like saying, "You're still my brother" to him. Dan shows love for Saladin even when he continuously scratches him in the first book.
Dan has dark blonde hair, a light complexion, and the same jade green eyes as Amy. He also has a small scar right under his right eye that he obtained from falling on his plastic AK-47 when he had gone commando raiding. It is frequently hinted that he is a bit small for his age.
He knows Morse code. He has a photographic memory. He is very good at math. He is very good with anagrams. He is a born puzzle-solver. He is good at figuring out mechanical things, like lock mechanisms.
CardsEditDan's main card is Card 2. He is also featured on other cards such as Card 250.
- He likes to collect items of all sorts, including a baseball collection worth nearly $3,000, autographs of famous outlaws, Civil War weapons, rare coins, his casts from broken limbs (apparently 12 since preschool), charcoal and wax tombstone rubbings, and samurai swords.
- His middle name is Arthur, after his father.
- He is fascinated with ninjas, but he didn't like being in the ninja fight in The Sword Thief.
- On the 39 Clues website, on the Cahill web, Dan is revealed to have stolen the heads of mascot costumes and even once glued a teachers desk to the ceiling during recess. How he glued it to the ceiling is unknown, especially in such a short time and without an adult there to help him.
- He attends Adams Middle School.
- He apparently still wears Pokémon underwear, as stated in The Black Book of Buried Secrets and it said in another book that he still wears Superman pajamas.
- He got in trouble three times on his first day of 7th grade, all resulting in him being sent to the principal's office, from the Black Book of Buried Secrets: 1.) Disrupted class, threatened teacher 2.) Disrupted class, talked back to teacher. 3.) Disrupted class, endangered students.
- He has asthma, and will often forget his inhaler or have Amy and/or Nellie hold it for him.
- Dan often uses "ninja moves" in school.
- He asked if he could do his history report on Benjamin Franklin's interest in farts. The teacher scolded Dan for disrespecting one of the founding fathers. Dan then replied that Franklin's greatest accomplishment could not be ignored by anyone.
- The teacher asked why Dan had missed the first three weeks of school, and he replied that "It's top-secret. If I told you, my family might have to kill you."
- He also said that he was a ninja lord, and demonstrated by back-kicking and accidentally hitting the fishbowl. Herbert the goldfish was said to be in critical condition.
- He brought Mozart's tungsten samurai sword in music class and waved it in the air, causing it to be confiscated.
- He thinks he is a ninja lord.
- On several occasions, he seems to think to use the Master Serum but Amy always stops him from doing it.
- Later on in the series, he becomes more responsible and starts to carry his inhaler with him, other than Nellie or Amy carrying it for him.