Kivat III started out with a fetish for beautiful necks. Normal behavior for a vampire bat, really.
Shima advising Megumi against taking on Rook by herself may be standard mentor behavior, but takes on added meaning in later eps when we learn that Yuri finally dealt Rook a Game-Breaking Injury, and despite Otoya leaving her, eventually found closure.
Ever noticed how the "organic" Riders require members of two different races in symbiosis (bats or whatever Sagarc is, with humans or Fangire or anything in between)? Kiva Emperor Form has a major advantage in three races in one, and Wataru could even be considered two races at the same time, which lends added meaning to the "Emperor" name, since an empire is formed by colonising and subjugating other countries with unified rule. Four is Death indeed.
The rat fangire from ep 36-37 could multiply. What are rodents known for?