First off, let's get one thing straight - while Tendou is quite obviously the main Rider of Kabuto, and the main mover and shaker of events, he's also very much a static character who exists mainly to move the plot along, be great at everything he does, and serve as a foil to numerous characters - but most specifically our main character, Kagami. For while Tendou is the one who primarily moves the plot along, it is Kagami who undergoes the most character development through the series and who is affected the most by the events.

From the very beginning Kagami is clearly meant to be our audience surrogate character - the character who we are meant to identify with the most and whose growth and arc we're supposed to be the most invested in - going from standing in Tendou's shadow to eventually being able to stand with Tendou as an equal, and maybe someday even being able to surpass him (see: Zi-O)

I didn't start thinking deeply about this until I ran across the fact that some schools of thought consider the protagonist of a story and the main character of a story to be two completely separate concepts. To quote:

What Is the Difference Between a Protagonist and a Main Character? The main character (sometimes called “principal character”) and the protagonist are both two central characters, but the protagonist drives the plot forward while the main character is impacted by the plot.

• A Main Character is the player through whom the audience experiences the story first hand.

• A Protagonist is the prime mover of the plot.

• A Hero is a combination of both Main Character and Protagonist.

In other words, a hero is a blended character who does two jobs: move the plot forward and serve as a surrogate for the audience. When we consider all the characters other than a Protagonist who might serve as the audience’s position in a story, suddenly the concept of a hero becomes severely limited. It is not wrong, just limited. The value of separating the Main Character and Protagonist into two different characters can be seen in the motion picture, To Kill a Mockingbird. Here, the character, Atticus, (played by Gregory Peck) is clearly the Protagonist, yet the story is told through the experiences of Scout, his young daughter.

TL;DR Going by these definitions and this school of thought, I think it's easy enough to conclude that in the case of Kamen Rider Kabuto, Tendou is the protagonist of the work whereas Kagami is the main character of the work.