Out of the entire body of Netflix's Marvel related work, The Punisher is likely both their darkest show and the show that is the most grounded in reality. All of the other Netflix MCU shows include some kind of magic or mysticism, but The Punisher seems to stick to the real world completely without ever addressing the more bizarre aspects of the MCU world.
So then, blending The Punisher's world together with something like the fantasy world of Harry Potter isn't necessarily the easiest mix. But considering the strange and magical things that have happened in the broader Marvel cinematic universe, it's not completely out of the realm of imagination that the characters of The Punisher could have wound up in something resembling the Harry Potter world. So if Frank Castle and the rest of his enemies and companions were sent off to Hogwarts School of Witchcraft and Wizardry for a top-notch magical education, which Hogwarts house would they have ended up in?
Lewis Wilson - Slytherin
Lewis Wilson is absolutely a skilled and dangerous soldier, but to be honest, he also has some delusions of grandeur and clearly sees himself as more important and more powerful than he truly is. He doesn't belong in Slytherin simply because he's one of the bad guys though. Slytherin is well known for it's elitist and exclusive attitude, and while Lewis does care a lot about the people he views as his contemporaries he clearly has zero regard for anyone who isn't a part of that little club. And Lewis is willing to do anything, however dangerous, to achieve his goals, which makes him very much a Slytherin.
John Pilgrim - Hufflepuff
There are very few people on earth who can step to someone like Frank Castle and even hold their own, let alone look like they might have an actual chance at victory. But John Pilgrim just so happens to be one of those people.
This mysterious character entered season two of The Punisher looking like the biggest and baddest villain on the block, but it turns out he was a lot like Frank and just willing to do whatever it took to save his own family. He's clearly a formidable warrior, however, his devotion to the ones he loves and doing his twisted version of the "right" thing makes him a great fit for Hufflepuff.
Krista Dumont - Slytherin
In theory, Dr. Krista Dumont was a psychotherapist helping trauma patients recover, but how effective of a therapist can you really be if you're a complete lunatic? Dumont excelled at creating an even-keeled facade to show the world, but her relationship with Billy and her total disregard for all of his destruction and heinous behavior just demonstrated how cold and self-centered she really was. Krista was undeniably a Slytherin through and through, and beneath the veneer of a kind and caring doctor was an incredibly twisted and cunning individual who seemed to have no regard for anything or anyone outside of her own desires.
Amy Bendix - Ravenclaw
Amy Bendix's naivete nearly cost her her life more than a few times during her time on the lam, but she's still one of the cleverest characters The Punisher has ever seen and undoubtedly would have been one of the brightest witches of her age if she had wound up at Hogwarts School of Witchcraft and Wizardry. Amy is definitely freakishly smart, resourceful, and seems to always find a way to land on her feet, but a Ravenclaw education undoubtedly would have made her life easier. Learning ways to channel all of her intelligence into something constructive would have been enormously beneficial for her.
Karen Page - Ravenclaw
Karen Page has been forced to adapt throughout most of her life in order to survive, and she hasn't really had anything to help her along with that aside from her own wits. She has repeatedly found herself thrust into the lives of some very dangerous people (whether or not they were villains or heroes) and while she couldn't match them in strength or resources she managed to survive or even best them by simply just outsmarting them.
Karen is actually cunning enough to roll with the Slytherin crowd as well, but her commitment to doing what's best for everyone outweighs her commitment to doing what benefits her, so she's really a Ravenclaw at heart.
Dinah Madani - Gryffindor
Dinah Madani's intelligence and ambition would actually make her a great fit in either Ravenclaw and Slytherin houses, however, she is committed to doing what's right when it comes down to the nitty-gritty of things which is why she ultimately belongs in Gryffindor. Madani is brave to be sure, and honestly, her arrogance would probably fit right in with most other Gryffindors too. And while Madani does value doing the right thing over everything else, she does tend to lose sight of that sometimes unless she's forced into high stakes situations, so being in Gryffindor may have taught her to keep her eye on the prize.
Curtis Hoyle - Hufflepuff
Curtis Hoyle's intelligence and morality makes him an amazing soldier, and it would have made him an amazing student at Hogwarts School of Witchcraft and Wizardry too. And while Curtis is an undeniably heroic man, he doesn't do what he does because he wants to play the hero or get a lot of attention and accolades.
His devotion to all things just and right are what make him the ideal Hufflepuff, and had he actually attended Hogwarts then he undoubtedly would have been one of the few students to bring a lot of attention and glory to the typically low key and humble house of Helga Hufflepuff.
David Lieberman - Ravenclaw
Micro has actually more than earned his spot in house Gryffindor if that is where he truly wanted to be, but his ability to literally outsmart the entire US government means that it would be kind of absurd if he wound up anyplace besides house Ravenclaw. David Lieberman almost died because he was so set on doing the right thing (so he'd actually be a good fit for Hufflepuff too), but he managed to both think quickly on the fly to save his own life and figure out how to play the most amazing game of 3D chess the world has ever seen in order to keep himself alive in the long run.
Billy Russo - Slytherin
Billy Russo is a character who could probably fit into any Hogwarts house outside of Hufflepuff, but given his ultimate goals and motivations in life he really belongs in Slytherin. He's brave and he's clever to be sure, but he only uses those talents to further his own agenda. And while there is a small part of Billy that cares for other things and other people, he would never let his affections for someone else prevent him from doing whatever he thinks is necessary to ensure his own success or survival, and he thinks he's pretty justified in doing so many awful things because his happiness matters more than anyone else's life.
Frank Castle - Gryffindor
If there has ever been any character on earth who had the brave heart of a Gryffindor lion, it's Frank Castle. If we're being honest, it would probably benefit Frank to have a bit of fear instilled into such an absolutely fearless man, however considering the fact that he has never been bested by anyone no matter how grim the odds looked it's actually pretty rational that he fears nothing and no one. Frank would not only fit right into Gryffindor because he's such an unstoppable badass, but a Gryffindor education would hopefully have been able to temper some of Frank's scarier and less self-preserving impulses.