It could still be Harry Potter himself. He has a history of facing dangerous creatures and dark magic, so it would be natural for him to take on the challenge of the deadly snake.
Another possibility is that a new character is introduced as the main character. This character could be a transfer student from a far - off magical school who has some special knowledge or ability related to snakes. They come to Hogwarts just as the deadly snake situation unfolds and end up being the key to solving the problem.
Another option could be a new character. A transfer student, perhaps, who is really a Slytherin at heart but is placed in Gryffindor by mistake. This character then has to keep up the pretense of being a Gryffindor while dealing with their true Slytherin nature.
Ron could also be the main character. He has his own courage and loyalty. He could be the one who has an unexpected connection with the poisonous snake, like being able to communicate with it in a special way that no one else can.
It could also be a snake from the Forbidden Forest. There are many unknown and mysterious creatures in there. A snake from that forest might be more in tune with the natural magic of Hogwarts. Harry, being the curious and kind - hearted person he is, might come across it during one of his adventures in the forest and start a friendship with it.
It could be Nagini. Even though in the original series she was more of a servant to Voldemort, in the fanfiction, she could have a different personality and form a bond with Harry.
It could be a dark wizard who also has the ability to communicate with snakes and wants to use this power for evil. He might be jealous of Harry's connection with the snake and try to disrupt it or steal the snake for his own dark purposes.
The main antagonist could be a Slytherin student who has a vendetta against Harry. When they see Harry with the snake, which they believe should belong to a Slytherin, they start causing trouble for Harry. They might try to frame Harry for using dark magic with the snake or try to steal the snake to prove their superiority over Harry.
It could be a dark wizard who also has an interest in the snake. Maybe he wants to use the snake's powers for evil purposes, like opening a portal to a dark dimension or controlling other magical creatures. So he becomes the main obstacle for Harry as he tries to protect the snake.
Maybe it's an anonymous gift from someone who has been observing Harry. This person could be a former student of Hogwarts who has a special connection to snakes and sees that Harry has a kind heart and would take good care of the snake. So they leave the snake for Harry to find with a simple note that just says 'For Harry Potter'.
One possible plotline could be the discovery of a new type of deadly snake in the Forbidden Forest that has some mysterious connection to dark magic. Maybe the snake is guarding a long - lost artifact.
Obviously, Harry Potter could be one of the main characters. He could be the one who has to face the challenges brought by the seven deadly sins. Hermione Granger would also be a great fit as she is intelligent and could use her knowledge to figure out how to deal with the sins. Another character could be Draco Malfoy. He could be initially tempted by some of the sins, like envy, but then have a character arc where he overcomes it.