In 'Horus Heresy', Horus is a key figure. Once a great and loyal primarch, he turns against the Emperor, leading to a galaxy - spanning civil war. There are also other primarchs like Roboute Guilliman who play important roles in trying to save the Imperium. In addition, characters like Loken, a Space Marine of the Luna Wolves, are central to the story as they witness the fall of Horus from the inside.
In 'Necromunda', Kal Jerico is a well - known character. He's a bounty hunter in the dangerous underhive. He has to deal with various gangs, mutants and other threats while trying to make a living. And in 'Gotrek & Felix', Gotrek Gurnisson, the Dwarf Slayer, and Felix Jaeger, the human swordsman, are the main protagonists, constantly seeking glory and adventure in a dark and dangerous world.
In 'Gaunt's Ghosts', the main character is Colonel - Commissar Ibram Gaunt. He's a complex leader who has to balance military strategy with the Imperium's strict ideology.