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?
Kiva is a rider that blends two races together. IXA is the human side, while Saga is the Fangire side.
IXA had four users in each timeline, each of them fitting a certain motif.
Nago/Otoya: Main users
Megumi/Yuri: Females, mother and daughter
Kengo/Jiro: Anti-Hero (Kengo was going through a punk phase)
Itoya/Rook: Fangires, they also used the power of IXA shortly before their death
Although other factors result in the movie being hard to placenote while the presense of Kiva's Emperor Form but lack of Rising Ixa form suggest that the movie takes place between episodes 24 and 27, Wataru did not know of the Arms Monster's existance or the time Door in Castle Doran until much later, and Megumi didn't learn that Wataru was Kiva until later either, Wataru going to school in the movie makes some sense, considering that he is a shut-in - he'd have to have left the house in order to have gone to high school, plus his job was one that didn't require a high-school education.