Disclaimer: I do not own Danny Phantom, as much as I would like to
Dani is age 7 in this I know she starts at age twelve in the series but I changed that to make her more of a little sister type to Danny, also Dani lives with his family
Danny was sitting on the roof in ghost form. He was looking out over town of Amity (or as much as he could see from the roof top) and thinking dark thoughts of Dark Dan, when he heard Dani fly up behind him.
"He wasn't you, you know," said the younger halfa as Danny turned around to face her.
"What?" Danny asked.
"Dark Dan. He wasn't you." Danny gave her a look and began to protest, but she cut him off. "Wait hear me out. Dark Dan was created from your ghost half and Vlad's ghost half, but think further than that. I think the parts of you that were actually you stayed with your human half. Vlad removed your ghost half to free it from the pain of losing your family, Right? Well the fact that the removed ghost half couldn't feel that pain shows that it wasn't you. You wouldn't have just stopped hurting like that. The ghost half was more like a body part. Not to mention Vlad's ghost half was in there too. So Dark Dan wasn't really you. Though, it's a moot point anyway, because thanks to a choice you made Dark Dan never existed and never will. That whole timeline is gone, replaced by a brighter future. Thanks to your choice there never was and never will be a Dark Dan. But even in the version of the future where he did exist he was never really you." Danny took this in and realized she was right. He felt like a huge weight had been lifted off his shoulders and a huge strain had been lifted off his mind. He couldn't even find the words to thank Dani so he just wordlessly pulled her into a grateful hug as a few tears of joy escaped him.
The next morning when Danny woke up he felt like he had a new lease on life, like a man who'd been granted a death row pardon because new evidence proving his innocence had arisen, like he had a fresh slate. Then, for the first time in a long time, he flew off just to enjoy the feel of flying. "Thank you," said Jazz who'd seen the whole thing through her window.
"A child's simple view of the world lets us see these things unclouded by guilt or blame." Said Dani, "And he's my brother too." And with that Dani transformed and flew off after Danny.