For the record, here's a list of franchises (including single game IP) that have not yet had a playable character added to Smash Bros. I would say a fair few are more popular than RH, and yes I realize that some don't really have a viable playable character:
*Only a few months old or not yet out
- 1080 Snowboarding
- Advance Wars/Famicom Wars
- Art Academy
- Balloon Fight
- Battalion Wars
- Bayonetta**
- Brain Age/Big Brain Academy
- Captain Rainbow
- Chibi Robo
- Clu Clu Land
- Codename: STEAM*
- Cubivore
- Custom Robo
- Daigasso Band Brothers
- Devil's Third*
- Devil World
- Dillon's Rolling Western
- Disaster Day of Crisis
- Doshin The Giant
- Drill Dozer
- Electroplankton
- Endless Ocean
- Eternal Darkness
- Excite
- Fatal Frame/Project Zero**
- Flingsmash
- Fluidity
- Fossil Fighters
- Freakyforms
- For Who The Bell Tolls
- Geist
- Golden Sun
- Gumshoe
- Harmoknight
- Hotel Dusk
- Mach Rider
- Magical Starsign
- Meteos
- Mole Mania
- Mysterious Murasame Castle
- Nintendogs
- Odama
- Ouendan/Elite Beat Agents
- Pandora's Tower
- Panel de Pon
- Picross
- Pilotwings
- Pushmo
- Rhythm Heaven
- Rusty's Real Deal Baseball
- Sakura Samurai
- Sheriff
- Shin Onigashima
- Sin and Punishment
- Soma Bringer
- Splatoon*
- StarTropics
- Steel Diver
- Style Savvy
- The Last Story
- The Legendary Starfy
- The Wonderful 101
- Tomodachi Life
- Trace Memory
- Urban Champion
- Wave Race
- Wild Gunman
- Zangeki no Reginleiv
**Partial Ownership only
Forgive me if I missed something
Famicom Detective Club should be on that list because it would be amazing. Music alone would be great, really.