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Danny isn't a Halfa, just an Extremely Liminal Human that has had a Ghost possess him, but neither he nor the Ghost knew until Danny accepted the Crown of Flames and the Ring of Rage. That's when he and the Ghost split due to the power the Crown and Ring provided. But it wasn't just any ghost that was Danny's other half. It was the Ancient of Space, who has been altered by his time with Danny. Before, the Ancient was a bit of an asshole, flaunting his power to the lesser Ghosts and constantly bickering with the other Ancients, other than Clockwork, who set up the entire thing to reform the Ancient.
Danny and Phantom (The Ancient wanted to keep the name that Danny had given him and not use his previous name) share the title of 'Ghost King', with Phantom keeping his title of 'Ancient of Space' and Danny gaining a new title, 'Space's Chosen'. Danny and Phantom have been getting closer and closer to each other, eventually they have become a couple.
One day, when Danny and Phantom are exploring the Infinite Realms, they come across a newly formed Ghost, one dressed in what looks like a Traffic Light Hero suit. Phantom immediately rushes to the Ghostling and dots on him, having already adopted the Ghostling as his son, Danny is more along the lines of getting to know the Ghost better then forming a parental bond.
Jason was the name of the Ghostling that Phantom and Danny found, and he's a Sidekick of a Hero in another dimension, but that's all he remembers, he doesn't remember the name of his family, or what he was doing before he died. Danny and Phantom are going to love and care for Jason, their Little Birdy, for as long as they remain.
That is, until Jason's revival, it happened during one of the family's outings in Amity, Jason was meeting Danny's living family and friends, a year after the Ghostling formed, with his Parents having accepted Danny's Ghost Half turned Boyfriend, when Jason is forcibly dragged into a mystical portal. Danny and Phantom went to Clockwork to see what has happened to their Ghostling. They are surprised to learn that he was revived by a Reality Warping event.
Danny and Phantom are going to Jason's dimension to find him and continue their little family life. Jason on the other hand, has been alive for awhile in his dimension, having mostly accepted his family again, he tolerates Bruce and is protective of his siblings, but he feels like he's forgotten something, or rather, someone's, maybe it's the two people he's having dreams about