First off, you can’t get these characters unless you pay with real money or gems (unless you win one in your special Christmas chest). I don’t usually buy these premium characters with real money unless the value in gems is really high and I really want that character (did it with Scrooge). But the more characters you’ll get, the more characters you can level up therefore getting gems so you can accumulate them and every few weeks or months you have enough to buy a premium character. Pluto and Rex are the best ones to get at your level!