While at Number 12 Grimmauld Place, Harry is the happiest he's been in ages. He's so happy, he even agrees to play hide and seek, even though it's childish. Whilst playing, the Weasley twins play a prank on Harry that reveals untold secrets. Abused Harry. Weasley/Hermione/Sirius/Harry bonding.
As Harry turns 15, he comes into a strange new inheritance. A scaly, dark, and very large inheritance. With great power comes great responsibility, right? Well if nobody knows about said great power... Then no great responsibility either, right? Harry quite easily manages to keep his secret from his Gryffindor friends...