Gavin MacLeod was the son of Crowley, whose destiny was to embark on a ship and die. Abaddon however traveled back in time and took him to the year 2014, where his father saved him from his fate until he chose to return in 2017 after meeting his fiancé Fiona Duncan as a vengeful spirit, restoring the timeline and letting things flow as they did before.
Gavin was born in Scotland in the late 1600s as the son of Crowley, then known as Fergus Rodrick MacLeod. He and his father hated each other, in life and in death, which stemmed from Fergus being an abusive alcoholic to Gavin. After his father's death, Gavin buried the latter and became captain of a trading ship known as the Star and left Scotland for America in 1723. He died soon after when his ship went down in a storm near Massachusetts. Gavin also had a fiancé named Fiona Duncan who he loved dearly and who wished to join him in his voyage to America.
Ocean Divers later discovered the wreckage and fished out, among other things, Gavin's signet ring. They donated it to the Naval Museum in Andover. The ring was later stolen by Rufus Turner, who attempted to deliver it to Bobby. Due to the police chasing him, Rufus swallowed the ring.
When Sheriff Jody Mills managed to get Rufus out of jail, Rufus gave Bobby the ring. From there, Bobby summoned the spirit of Gavin and they talked. Gavin had no love for his father and told Bobby everything (including how Crowley became a demon). Crowley and Gavin met again when Bobby summoned him. Crowley initially pretended to get emotional, but then quickly admitted that he hated Gavin. He then told Bobby he could torture Gavin all he wanted, but Bobby informed him that Gavin wasn't a hostage. Crowley was shocked when he found out that Gavin told Bobby where Crowley's body was buried. Having gotten his revenge on his father, Gavin vanished.
In King of the Damned, Abaddon traveled back in time to 1723, taking Gavin to the year 2014. Later, she uses him as a chip assuming Crowley has some humanity, having been injecting himself with purified human blood. She tortures him for a while, clenching him in a telekinetic bind when Crowley finally gives in.
Gavin initially does not believe Crowley is his father, or that he really time travelled. Crowley then shows him a light bulb, while Abaddon shows him outside. They realize Gavin is not very smart, and Gavin blames this on his father. Crowley then reveals to Gavin about his demon deal and requirement to use a vessel. Gavin is baffled at first, then insults Crowley for all the mistreatment he gave.
He is however, a little excited over the prospect of being "Prince of Hell" (albeit no powers). Sam and Dean eventually defeat Abaddon and insist that Gavin must be sent back to his own time, in order to avoid a distortion in history and time. Instead Crowley teleports him away, however, and tells his son of his fate. He then instructs him to start a new life in the 21st century since he now had the chance.
In Family Feud, after discovering that a vengeful spirit that is killing people is from Gavin's old ship the Star, the Winchesters locate Gavin with the help of his grandmother Rowena and contact him, claiming that his father is sick and needs his help. Gavin arrives on a bus and the Winchesters explain what is happening to him after introducing Gavin to Rowena and have him look through the artifacts from the Star. Gavin is horrified to recognize a locket in the inventory as belonging to his fiancé Fiona Duncan who was supposed to meet with him before Abaddon abducted him to the present. Gavin realizes that with him gone, Fiona must've stowed away aboard the ship and it's discovered that Fiona is the vengeful spirit.
After saving a teacher from Fiona's attack, Gavin uses a séance to contact her. Fiona appears before Gavin and explains that without Gavin there to protect her, when she was discovered on the Star she was brutalized and raped by the crew. She now wants revenge on teachers as their teacher encouraged the treatment instead of stopping it. Discussing options, the Winchesters suggest sending Gavin back to his own time which will negate Fiona becoming a vengeful spirit as Gavin will be there to protect her and undo her murders. Gavin agrees, having already come to the same conclusion and feeling that he'd never fit in in the 21st century. Crowley arrives having been called by Gavin to say goodbye and attempts to stop them. Gavin refuses, feeling that it's the right thing for him to do and says goodbye to his father after Rowena immobilizes him.
Gavin returns to the Men of Letters bunker with the Winchesters who prepare a version of the Blood Sigil modified by Rowena to return Gavin to his time. As Sam casts the spell, Gavin holds Fiona's locket and her ghost materializes next to him. Gavin and Fiona embrace as the spell returns them to 1723. This restores the original timeline, and Gavin dies on the ship.
After Gavin is gone, the Winchesters find that all of Fiona's victims are still alive, showing that he succeeded. Rowena later admits to Crowley that sending Gavin back to die was in part for her to get revenge for Crowley forcing her to kill Oskar to remove the Mark of Cain.
Gavin as a human was shown to be quite naïve. He was a slow learner and did not know how to read; both are the result of his father's decision not to let him study when he was younger. Gavin held contempt for his father for his brutal treatment of him, contempt which appeared to have intensified following his death.
However, Gavin was not above forgiving the man he had hated all his life. Following Abaddon's death, Gavin acknowledged that his father had changed and was willing to hug him had his father, now known as Crowley, not backed away.
Gavin may have been religious, given his belief in the existence of angels and Heaven. Despite his harsh childhood, Gavin had high hopes for his own future, only to become devastated when he learned his dream as a trading ship captain was futile, and would end in his own demise. He was also excited about becoming the Prince of Hell, something very few people would find pleasant.
Gavin loved Fiona Duncan deeply to the point of calling her the love of his life and being willing to return to 1723 to die just so that he could spare her a horrible fate. Rowena MacLeod notes that unlike herself and Crowley, Gavin actually believed in something and was willing to do the right thing simply because it was right. While speaking with the Winchesters, Gavin notes that he was never able to truly fit in in the 21st century.
Gavin is a tall young man with black hair and dark eyes. His skin was fair. Gavin spoke with a strong Scottish accent. His ghost, which was summoned by Bobby, had a different appearance compared to the version brought back through time. This Gavin had short hair and round eyes and resembled mostly Crowley's vessel. The version brought back in 2014 wore a typical outfit of 16th century Scotland, with a white shirt, a black vest and dark-colored trousers. His hair was messy and he bore facial hair. A few years after trying to adjust to life in 21st century, Gavin wore a light blue shirt with a dark blue coat and light brown trousers. His hair was kept neater and short this time.
According to his grandmother Rowena, Gavin looked strikingly similar to his paternal great-grandfather when the latter was young.
- He is the only known offspring of a demon from the demon's time as a human.
- Azazel claimed Meg and Tom were his children, but it is unknown if he meant this literally.
- In a deleted scene for Family Feud, Gavin and Rowena briefly talk about their family history and Gavin reveals that Crowley visited him from time to time and even gave Gavin money to live on until he got a job.
- Interestingly, Gavin's displacement to the 21st century did not seem to effect Bobby Singer's deal with Crowley and Bobby's efforts to break it. To break the deal, Bobby summoned Gavin's ghost to find Crowley's bones, capitalizing on Gavin's hatred for his father. Gavin being brought to the 21st century would've removed Bobby's ability to call upon his ghost in the alternate timeline and when the original timeline was restored, Gavin had lost his hatred for his father due to his experiences in the present.
- Its possible the Winchesters informed of this before sending him back to his own time so Gavin pretended when Bobby summoned him to keep history on track.