A budding concept I have for Harry, before being possessed by Sol...
There is no Cinders. It was Harry’s old name. He lied completely about his mother giving him up, the entire story was fabricated to evoke sympathy from Leafstar. The truth is that those were his own kits.
Sol approached him after his mate left, in the form of a koi fish that Harry couldn’t catch. Every time he struck at it, it would slip through his claws as if it was water. When he gave up, it spoke to him, flattering him by calling him beautiful (”I’ve always loved the fur of a calico”), offering a partnership,
“But first, I need to know you can be faithful. I need three.“
“...three of what?“
“Sacrifices of course.“
It takes some time, but Harry decides that power is worth it. To have the changes in his life he’d always wanted... but he only had two kits to give. He needs a third, and THAT was what SkyClan was about. He joins SkyClan planning to snatch Leafstar’s kits away... and for a brief time, is tempted by warrior life. New start, a life of less hardship, with community...
But of course his plans backfire. He isn’t cut out for the life. But this time he refuses to lose everything, he only needs ONE last kit, and he manages to abscond with Firekit or Stormkit. The third sacrifice, and that’s when Harry is possessed by Sol.