Definitely his professors when they were younger. For example, Snape as a student. This would be a strange interaction considering their relationship in Harry's own time. He could also meet Hagrid when he was just starting out at Hogwarts. And there's a good chance he'd run into Sirius and Remus during their school days, and perhaps form new friendships or alliances with them in the past.
Obviously, he would interact with his parents, James and Lily. He could also meet a young Snape and try to change their relationship for the better. Dumbledore in his younger days would be another character. Harry might seek his advice or try to understand his earlier actions. Hagrid as a young gamekeeper would also be someone Harry would likely cross paths with.
Hermione would probably be one of the main characters to interact. She might be horrified at first to see Harry in such a situation and would try to find a way to free him. Ron could also be involved, perhaps being more impulsive in his attempts to rescue Harry, which might cause some problems. Dumbledore too would likely play a role. He might have some knowledge about the situation that he's not sharing right away, and his interactions with Severus would be crucial in determining Harry's fate.
Voldemort would be a likely character to interact with Harry. Since Bellatrix was one of Voldemort's most loyal followers, he might take an interest in Harry as her son. He could either see Harry as a potential powerful ally or a threat depending on how his powers develop. Another character could be Draco Malfoy. Given the connection between the Malfoy family and the Death Eaters, Draco might have a complex relationship with Harry, perhaps initially seeing him as an equal in the dark magic world but later having doubts as Harry shows signs of not fully embracing the dark side.
Obviously Gibbs himself would be a main character interacting with Harry. He'd be like a father figure.
Snape would have a very different relationship with Harry in this scenario. Instead of being a constant source of disdain, he might see Harry as an opportunity to shape a Slytherin in a new way. Pansy Parkinson could also be involved. She might initially be against Harry being in Slytherin but later come to accept him, or they could be in a constant battle of wits.
Hermione and Betty would be likely to interact. Hermione's intelligence and thirst for knowledge could pair well with Betty's investigative nature. They could work together to solve a mystery that spans both the wizarding world and Riverdale.
Neville could be another character. He might look up to Harry even more in his dragon form, seeing him as an even more powerful protector. Hagrid would be excited to have a dragon - Harry around. He could help Harry understand his new dragon nature. And some of the Slytherins, like Draco, might be jealous or scared of Harry's new form.
There could also be some minor characters that become more important in this fanfiction. For example, Harry's parents, James and Lily. Since Harry is changing the past, they might have more of a role to play. Or perhaps characters like Sirius Black or Remus Lupin could be involved in different ways due to the changes Harry makes in the past.
In this fanfiction, it's likely that Harry and Gabrielle will interact with some of the Weasley family members. Since Gabrielle is Fleur's sister, the Weasleys could play a role in how the life debt is dealt with. They might offer advice or resources. And of course, they could also be involved in any conflicts that arise. Also, there could be interactions with some of the Hogwarts professors, who might have their own insights on life debts and how to handle them in the wizarding world.
Obviously, he'll interact with his main friends like Ron and Hermione. Ron might be both amazed and a bit jealous of Harry's new Sans powers at first. Hermione would probably be very curious and start researching to figure out where these powers came from. Harry might also interact with Dumbledore. Dumbledore would be intrigued and might offer some guidance on how to control and use these powers for good.