Nintendo has confirmed that Kamek and Dry Bones are coming to Mario Tennis Aces in the very near future. Kamek will arrive in April and Dry Bones is coming in May. If you're not planning to grab them online, you will have to wait a bit longer.
Both characters are well known to every Mario fan (and Nintendo fan for that matter). Kamek first appeared in Super Mario Bros. 3 while Dry Bones made a debut in another legendary game - Super Mario World. Both characters are loved by the fans, so there is no doubt that Ninty made the right choice by adding them in.
Hopefully, this will make you play Mario Tennis Aces if you haven't already. It's really good!