It just so happens that today's Hot Week box gives a free character slot.
Other ways to get a free character slot are Fairy Bros (click "Daily Gifts" in the menu that pops up from the … (more)green "calendar" button at the bottom of your screen), and various events and event coin shops (none available right now, because we're winding down the current batch of events. Look for new ones starting in about a week). (less)
