Is this for pixel art? Generally this is solved for pixel art games simply by designing them so that you dont have to scale characters. It's something I learned early making games with AGS, that scaling pixel art never looks great. So now I always avoid scaling up, prefering to shrink down the room instead as a cut-away. And I tend to do room art with the character in them so I can avoid too much scaling down too.
For hi-res games, you also have scale control on the sprite importer, so that might be what you're looking for.
For the other scaling issues, it's hard to know what you mean without screenshots/vid maybe?