There’s a moment in Prisoner of Azkaban – after Harry has discovered his godfather Sirius is not a murderer, but before Sirius has been recaptured – when a home with a real parent-type figure seems on the cards. Living with Sirius is an idea Harry jumps at, despite having, until very recently, believed Sirius to be a very Dark wizard. Which is a mark of just how lacking Harry’s home life has been – something Sirius understands. They have a lot in common…

Both had miserable childhoods

Having lost his parents at a young age, Harry was brought up by a bunch of the biggest Muggles imaginable. Unaware that he’s a wizard, Harry has to deal with the Dursleys’ dislike and a contempt so deep it has him sleeping in a cupboard. While he’s fed, schooled and free from physical pain, Harry grows up without the love of family or the companionship of friends.

On the other hand, Sirius grew up with two parents and a brother, Regulus, but he was so different from the pure-blood-loving Blacks that he ran away at 16, an action his mother rewarded by removing him permanently from the family tapestry. The mumblings of Kreacher, Sirius’s very begrudging house-elf, reveal Sirius was hated by his mother, and the feeling was mutual.

Both find comfort in other people’s families

Harry meets Ron on the Hogwarts Express and they bond immediately – much like when Sirius met Harry’s father James. While the Weasleys become the family Harry never had, it seems the Potters did the same for Sirius.

Harry becomes so firmly embedded in the Weasley family that Ron’s mother Molly’s response to the reminder that Harry is not her son is: ‘He’s as good as.’ Similarly, when Sirius left home it was James Potter’s family he escaped to. ‘They sort of adopted me as a second son,’ Sirius says.

They’re both famous

Harry has always been The Boy Who Lived and also becomes known – if not believed – for his insistence that Voldemort has returned. Meanwhile, Sirius is infamous for being the man who escaped Azkaban and, wrongly, Voldemort’s most loyal supporter.

With both of them facing distrust from – and, in Sirius’s case, reimprisonment by – a corrupt Ministry of Magic, they are also subject to speculation from the Ministry-influenced press. For Sirius this means he is literally housebound and, after a long summer at the Dursleys’, Harry also knows the frustration of being kept away from important matters for his own protection.

Both have suffered loss

With the death of his parents, Harry lost the opportunity to grow up in a loving family. But Sirius also suffered, losing a friend who was like a brother. Both of them have their lives literally changed by Voldemort’s actions: Sirius spent 12 years in Azkaban for a crime he didn’t commit and Harry spent 11 years at the Dursleys’, protected but neglected, deprived of love and friendship.

They’re natural leaders

Perhaps those experiences of unhappiness, grief and imprisonment go some way to explaining the similarities in Sirius and Harry’s temperaments. Both hate to be idle. It’s obvious in Order of the Phoenix, when they’re each kept in a kind of limbo by Dumbledore, but it’s not just enforced protection they rebel against.

Within their respective friendship groups, both Harry and Sirius are ringleaders. Harry, for instance, refuses to listen to McGonagall when she tells him the Philosopher’s Stone is well protected, and Ron and Hermione join him to retrieve it. What we know of Sirius’s school years paints him as somewhat less noble – encouraging James to hex Snape because he’s bored, for one – but, like Harry, he is pretty influential, with even James keen to impress him.

They’re both rule-breakers

All of this means neither has a problem with breaking rules. When he escapes Azkaban, Sirius – supposedly keeping a low profile – turns into a dog to track Harry and even breaks into Hogwarts. Harry has a similar disregard for his own safety. On many occasions – with the Philosopher’s Stone, and when he risks discovery by Umbridge to check on Sirius – this is motivated by what Hermione calls his ‘saving-people thing’. But his recklessness isn’t always noble. He does like to roam the corridors in his father’s Invisibility Cloak, and he wasn’t above sneaking into Hogsmeade when he was supposedly being hunted by his parents’ murderer.

This is behaviour Sirius indulges, even encourages, telling Harry: ‘The risk would’ve been what made it fun for James.’

Sirius is pleased both by how similar he and Harry are and how similar Harry is to his father. But, as Molly reminds Sirius, Harry is not James – and even Sirius has to acknowledge that, at 15, he and James were ‘arrogant little berks’. For all their similarities, this is where he and Harry differ – they may share a tendency towards overconfidence and rash behaviour, but the 15-year-old Harry is far more likely to listen to reason than Sirius ever was.