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 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).
Kanna's main stat is INT. But HP gives her M.Att, which adds to damage range as well.
Depending on how much INT, %INT, HP, and M.Att you already have, %HP can give you a significant boost.
There is currently an issue with using mount coupons. I don't think it's related to how many mounts you already have. The collection is supposed to be "infinite".