Kenji Matsuda appears in Makai Senki as a blue Makai Knight with a curved sword named Wataru. Compare Kamen Rider Kiva, where he appeared as a blue werewolf who had the ability to transform into a curved sword for a young man named Wataru.
The Danza: Rei Suzumura is portrayed by Ray (pronounced the same) Fujita.
Hey, It's That Voice!: Ai Orikasa not only provides the voice of Rei's familiar/amulet Silva, but she also shows up in person when Silva appears in human form in the gaiden episode "Smile". She also plays Luckyuro
Along with Hironobu Kageyama, who's been voicing Zaruba AND singing the theme song.
The powerful Horror who served as a MacGuffin in Makai Senki is named Ganon.
The new character from the Soukoku no Maryu movie is named Kakashi.
You Look Familiar: The theme park manager from Episode 10 of the 2005 series (who harasses Kaoru) shall later on play Ratess in Makai Senki.
The new character from the Soukoku no Maryu movie, Kakashi will be played by Yuki Kubota, who previously played the obnoxious idol/actor JURAN in episode 9 of Makai Senki. He was the episode's Victim of the Week.
And now, Kouhei Otomo, who played Igari Juzo in Makai Senki is playing Burai, the old Makai Priest in the 3rd season.
Takano Hassei, the son of the old couple in Makai Senki #6 (see also Hey, It's That Guy! above) eventually became The Quisling to the Horror Ring in ZERO: Black Blood.
Thane Camus, who plays Ring himself in ZERO: Black Blood, is set to re-appear as another Horror in Episode 5 of Makai no Hana.
Keiko Matsuzaka, who played Big Bad Judam in Soukoku no Maryu, reappears as Anna (Gonza's chef girlfriend-of-sorts) in Ep. 10 of Makai no Hana.