A Little bird told me
Danny knew his dimension didn't need him. It had been a long time since it stopped needing him, a long time since he had to be content to spend most of his days in the Infinite Realms. His sister and friends were too busy these days to pay attention to him; Danny understood, it's not like he could work or have a job, considering he'd stopped growing at 21 and people would start asking questions eventually.
Honestly, it was a relief that he'd even been able to grow up to that age, Clockwork's knowing look told him he had help with it.
So, he distracted himself by learning things from the other ghosts in the Realms, who were definitely as bored as he was. He even managed to get Vlad to teach him duplication, but it wasn't that interesting after a while. Though he had become interested in the different dimensions that Clockwork watched over.
The problem was that there was one dimension that had caught his attention (one full of heroes and magic) but they always made the worst decisions. There came a point where he decided to interfere, Clockwork seemed amused so he figured he wasn't going to stop him.
As he thought about how to infiltrate (definitely not as a hero, he loved his retirement, thank you very much), he remembered a rather...odd power he had recently discovered.
Danny had discovered that he could shapeshift. The problem was that he could only shapeshift into dead animals and well, while it was fun to scare others, he didn't know how well people would take a ghost crow with ectoplasm coming out of it's wounds.
Figuring it was better than nothing, he transformed into a bird and flew through the portal; he flew towards John Constantine, who seemed fed up with his life. Constantine knew the bird was fucking weird the moment it sit on his shoulders but he had better things to take care of, like the demon in front of him.
Said bird apparently knew the way to defeat the demon, because he started naming the ingredients needed to banish it. Constantine saw it with narrowed eyes and asked if he wanted his soul, the bird pecked him, looking annoyed.
From there, seeing that the dead bird was doing no harm, Constantine let it stay. It was oddly useful and he had sold his soul for less.
Danny spent his days whispering things to Constantine to defeat enemies and the hellbazer gave him cookies in return (the halfa really wanted to be offended), when the League saw Constantine with a dead crow on his shoulder they wondered if he had finally lost his mind.
John commented that his name was Ghosty (he was pecked again) and that he was useful, unfortunately for the superhero community, Constantine had never been that useful and therefore they couldn't complain (but why did he suddenly know all the existing gossip?, he kept bribing them with it! His crow looked amused too).
Every time Constantine won a battle without explanation, someone would make the mistake of asking how he did it. With a shit-eating grin, Constantine would point to his shoulder and say "a little bird told me."
Danny was so tempted to shape-shift just to bite his head off, but the cookies were good.