Each familiar has two abilities. The ability to turn into 1 animal and another special power. Harry's special power is a manymagus. Basically, he can turn into any animal, however any injury sustained while in his animal form is transferred to his human form and he turns into his human form when he's injured enough.
In this story, Tom Marvolo Riddle will be a girl. This is so she can still be insanely powerful and also so we can possibly have a Harry/Rose/fem!Voldemort pairing. (Harry/fem!Voldemort is one of my favorite ships and it's super difficult to find so my hope is y'all will vote for it :D)
Ok, so here's what happens between book 1 and book 3.
In book 1, Harry and Rose go to Hogwarts and are sorted into Hufflepuff (They're super loyal to each other, plus I'm a Hufflepuff so I'm slightly biased lol). Rose quickly befriends Susan and Hannah. Harry tries talking to the other Hufflepuff boys but doesn't get along with them and ends up joining Rose and befriending Susan and Hannah at the end of the book, we find out that the teacher of Familiar Arts, Professor Quirrell, has Lady Voldemort stuck to the back of his head and Harry, Susan, Hannah, and Rose defeat him. Throughout the book, Rose and Pansy become arch rivals and hate each other.
In book 2, Harry and Rose get to Hogwarts and everything starts off well, then people start getting Petrified. Everyone assumes it's Harry, (Since he can turn into animals so they assume it's him turning into a Basilisk.) and Harry's only friends are Rose, Hannah, and Susan. Then Susan gets petrified and Hannah and Rose blame Harry. Harry is bullied and alone, then he is approached by a Slytherin girl, Tracy Davis. Tracy and her best friend Daphne befriend the lonely Harry and the three of them go into the chamber of secrets. Harry turns into a Basilisk and kills the one under the control of Lady Voldemort, although he is bitten in the side. He turns back into a human and is dying of poison, when Fawkes flies in and cries on him, saving him. The three people plus Fawkes fly out of the chamber riding on Fawkes, but in all their worry about Harry, they forgot about the Diary, which, although still alive, is unable to move itself and that Horcrux is now trapped in the chamber. Things are awkward between Harry and the other Hufflepuffs, especially Hannah and Rose, who realize they were wrong. Susan wakes up and discovers what happened to Harry and is livid with her two friends, but talks to Harry and understands why he's a little hesitant to strike up a friendship again with Rose and Hannah. Susan and Harry remain friends but Harry now spends most of his time with Daphne and Tracy.
In book 3, Rose and Harry are still awkward around each other. The book mainly goes the same as it does in canon, except instead of Harry Ron and Hermione going into the whomping willow,, it's Harry, Daphne and Susan. Sirius Black has been in Azkaban so the Dementors have completely neutralized his powers, leaving him with only his animaus skill. Susan is the one that gets her leg injured and Daphne and Harry use Daphne's time turner to go back in time and stop the execution. Harry turns into a phoenix and uses the fiery light magic to scare away the dementors, but passes out after.
The summer goes much the same and Harry and Rose arrive at Susan's house towards the end, along with Hannah where the next chapter will start. Things are still awkward around Hannah, Rose, and Harry and Harry hides in his room, mainly talking only to Daphne.
So that's the summary! I'll try to get a super long chapter out as soon as I can! I'd love it if you guys can review and tell me who you think the third girl in the Harry/Rose/? Pairing should be! But just to keep it interesting, I won't tell you who the third person is! So you'll just have to read and find out! R&R please!