"'Bernie Kosar, you made it through. I owe my life to you.'" -John Smith, I Am Number Four
Hadley, commonly known as Bernie Kosar and usually abbreviated to BK, is Number Four's Chimæra. He is presumably the only Chimæra to have been on the ship transporting the Nine Garde Children to Earth.
Hadley was born and raised on Lorien by Number Four and his family. He stayed with Four and his grandfather while Liren, Four's father, went off to master his Legacies. He went with Four and the other eight Garde on the ship, and John remembers playing on the ship with what he thought was a stuffed animal, before also remembering that the animal used to play back.
After landing, Hadley took on various forms, watching over Four in secret. John finally notices that he is being watched when he sees a gecko following him everywhere when he lived in the Florida Keys, but was not aware that it was Hadley. In Paradise, Ohio, while John is meeting Sarah Hart, Hadley appears at the high school in the form of a beagle, and then later appears at their house while Henri is training John with his Legacy of Lumen. They take Hadley in and name him Bernie Kosar.
BK stays with them throughout their time in Paradise, always patrolling the lawn in the morning and going on runs with John. Albeit he is never able to catch up to John in his beagle form, he secretly transforms into a bird and flies ahead of his Garde, appearing right in front of him. John used to just think of this as strange, not realizing that his dog was shapeshifting.
Later, BK is seen at the battle of Paradise High. He is apparently ready for combat, growling at the windows and standing on his hind legs. Number Six almost reveals what BK really is, but Henri stops her, wanting John to figure it out on his own. He kills a Vatborn Mogadorian in his beagle form by biting it in the neck.
When John is cornered by a piken, BK finally reveals himself as a Chimæra when he communicates telepathically with an injured John, telling him he will fight, and transforming into a massive beast, John stating that he "looks like a dragon, but without the wings." Albeit BK is able to fight the piken, he is severely injured and almost killed by the beast. BK is unconscious and unable to fight the rest of the battle, and wakes up next to John in a motel after they win. He goes with Sam Goode, John, and Six when they leave Paradise and go on their search for the rest of the Garde.
BK is a great help to them on their journey. Despite recovering from his injuries for a while, BK manages to be of aid; he turns into an eagle to lead hounds away from them, and turns into a "giant elephant-bear animal" to help Six when she almost gets killed fighting the Mogs after they return to Paradise. Prior to that event, he swims laps with Six in the form of a platypus-like creature. When John and Sam go to infiltrate the West Virginia base, BK sits outside, since the gases inside the mountain will kill him. After John leaves the base with Number Nine, he transforms into a horse and helps them escape.
Because Nine also shares John's Legacy of Animal Telepathy, BK and Nine begin to interact as well. He even shows Nine the West Virginia base, telling him information about infiltrating the location even before he tells John. When they are attacked by the FBI agents under orders of MogPro, he turns into a cockroach and gives them help on how to escape.
Once they arrive in Chicago, BK repeatedly tries to keep John and Nine from fighting each other. However, he fails to do this twice, John losing both of the fights and BK being disappointed in them both. Hours later, the two Garde agree to head to New Mexico and save the Garde. He turns back into his beast form to help them fight the MogPro agents stationed outside the base. BK gets injured by Mogs, but survives to fight another day.
BK accompanies the rest of the Garde to Chicago. He goes with John, Six, and Sarah to find Number Five. While there, BK transforms into a panther to fight a giant worm monster unknowingly created by Five. When they return, he trains with the Garde, turning into a giant boa constrictor during a game of capture the flag.
When the Garde are checking their Loric Chests for Phoenix Stones, BK is extremely infatuated with the Phoenix Stone belonging to Number Eight, a curved antler which he claims is a totem for the Chimæra. Later in the night, BK begins to act primal, making a loud animal call and wrestling the totem from the Garde. According to John, "He says he was calling the others."
After John and Ella come down with a strange telepathic illness, he stays with them, never leaving John's side until the Mogadorians attack. He turns into a bear and fights them, but is nearly killed and left for dead by the Mogs on the roof. John goes to save him, finding Adamus Sutekh and the other Chimærae along the way.
BK is making a decent recovery when John and the other Chimærae are in Baltimore. After learning that a Loric supporter may be Mark James, Sarah goes to find him. BK goes with her. Arriving in Alabama, he greets Mark in the form of a giant bird. He later holds off the Mogadorians when they overrun Yellowhammer Ranch before running off with Sarah and Mark. They make it to Atlanta, and meet GUARD, a secret Loric ally. At first they believe they are falling into a deadly trap, and BK begins to grow hostile toward a masked figure. The person speaks to him in Loric, commanding him to heel. The woman reveals herself as Lexa, a Cêpan who escaped the Planet Lorien.
Lexa flies them down to the Sanctuary, where he greets Six, Marina, and even Adam. They plan their ambush against Setrákus Ra and their attempt to commandeer the Anubis. The plan fails, but BK still fights the Mogadorians, defending his friends. He turns into a griffin to help Six when she fights a vulnerable Setrákus Ra. Despite being cut by a sword that Ra slashes him with, BK still survives. At the end of the fight, he watches as Sarah slowly fades away, and whines while licking her cheek. She dies, and he feels guilty for not being able to protect her as John commanded him.
BK stays by Sarah's side with Mark, mourning. He later comforts Six when she returns to the recreation area. When they arrive at Patience Creek, John steps aboard the ship, and tells BK that it was not his fault that Sarah died. He continues to mourn her, however.
When they go to commandeer the warship Delta, BK and Dust go aboard with John, Six, and Adam to take out the Mogadorians inside. He and Dust transform into monstrous versions of their normal forms, backing John as he uses his Legacy of Ximic to rip through the warship.
He goes into the West Virginia base for the final battle, and kills a piken along with several other Mogadorians in his griffin form. When they face a highly augmented Setrákus Ra, BK is captured in a cage of the black oil, taking smaller and smaller forms with the cage until it shrinks into a cocoon, sealing him away. Six frees him, and he escapes the cave with an incapacitated John and Adam, before the base is bombed by Sam.
A year later, BK is still alive and well, living with John in the Himalayas.
Bernie Kosar is first seen as a lizard following Henri and John when they leave Florida. He travels with them to Ohio after sneaking into the Loric's luggage. When John and Henri arrive at their new house in Paradise, Ohio, Bernie shapeshifts into a beagle. John finds the beagle and dubs him "Bernie Kosar" after the Cleveland Browns football player. Bernie follows John to Paradise High School on John's first day and witnesses the teen entering the school.
- Bird - BK used this form back on Lorien while playing with John, and also on Earth when he would meet go on runs.
- Gecko - This is the first form we have seen Bernie Kosar in when Four and Henri were living in Florida.
- Beagle - He was in this transformation when he first met Four and Henri and got the name Bernie Kosar
- Wingless Dragon - Bernie Kosar used this transformation to battle against the Mogs at the Paradise, Ohio high school in I Am Number Four.
- Bearded Platypus Beast - This form was used when Six was swimming laps in their pool in Florida.
- Giant Elephant-Bear Monster - Six described BK in this form when she was left alone at Sam's house in Paradise.
- Horse - This form was used after John and Nine broke free from the West Virginia Base.
- Cockroach - Used when Nine and John were captured by Agent Walker.
- Chameleon - BK used this form before they went to Nine's penthouse.
- Panther - A form BK used when fighting one of Five's creations. It appears that this is a common attack beast used by Chimærae.
- Grizzly Bear - This was the form BK used in Nine's penthouse. He was severely injured, and lost part of his ear in this form.
- Griffin - Bernie Kosar used this battle form against Setrákus Ra in The Fate of Ten and United As One.
- Dog Beast - This is the form BK used when he and Dust took control of the Delta.
- Tiger with a Ram's Horns- This is the form BK used when 4, 6, and Sam went to Sam's house to find a transmitter. The Mogodorians ambushed them and BK used this form to fight them.
- Bernie Kosar is named after the Cleveland Browns football player of the same name.
- The name of the beagle that portrayed Bernie in the film is unknown.