Mephiston is also very important. He is a Librarian with extraordinary powers. His character is complex as he battles with his own inner demons while using his psychic abilities to aid the Blood Angels in combat.
One great one to start with is 'Dante'. It gives a deep look into the character of Dante, the Chapter Master of the Blood Angels. It's full of action and shows the unique culture and traditions of the Blood Angels.
One of the main characters is Ciaphas Cain. He is from the 'Gaunt's Ghosts' series. He is a bit of a reluctant hero, always getting into and out of trouble in the most interesting ways.
Marshal Helbrecht is a main character. He is a powerful and respected leader within the Black Templars. His decisions often shape the course of their crusades.
In many of the novels, Asdrubael Vect is a significant character. He is a powerful and cunning leader within the Dark Eldar society.
In 'The Knight of Talassar', Sir Hekhtur is a main character. He is a noble and skilled Imperial Knight who has to face various challenges. His character is well - developed, and we see his growth throughout the story.
There are many main characters. Horus, as I said, is a central figure. The Emperor, of course. Then there are the various primarchs. For example, Sanguinius, the angelic primarch of the Blood Angels. He has his own story arc of loyalty and the struggle against the heresy. Also, Fulgrim of the Emperor's Children. His fall to Chaos is quite a significant part of the narrative. And Guilliman, who later plays a huge role in trying to hold the Imperium together after the heresy.
Azrael is a main character. He is the Supreme Grand Master of the Dark Angels and plays a crucial role in leading the chapter.
Dante is a main character in many Blood Angel novels. He is the Chapter Master of the Blood Angels. He is a powerful and respected leader, guiding his brethren through many challenges.
I don't know for sure, but it could be the Space Marines. They are often central characters in Warhammer 40k stories.
The Empress herself is of course a main character. Often, you'll also find Space Marines, as they are a significant part of the Warhammer 40k universe and interact with the Empress in various ways. Imperial Guardsmen may also be present, depending on the plot of the fanfiction.