The primarchs

Intro  Angron is angry, but a lot of that is the Butcher's Nails. I think the most interesting interpretation of his personality is that he has a bad case of PTSD based on his failed gladiator rebellion on Nuceria and how it ended. Read Betrayer. Fulgrim is a perfectionist, but he's somewhat insecure. Read Fulgrim. Guilliman is in control. He's incredibly self-possessed and rational almost to a fault. Read Know No Fear. Horus is proud. It's both his strength, when it's justified, and his weakness, when it's threatened. Read Horus Rising. Jaghatai is thoughtful, but when he acts he does so quickly and decisively. Magnus is inquisitive and impulsive. He wants to know everything, and damn the consequences. Read A Thousand Sons. Perturabo is just mad at the world. All he wants is to invent and build, but his job is to tear everything down. And even when he does it well, he doesn't get the respect he wants. Read Angel Exterminatus (I've heard the recent Perturabo is good, but I haven't read it, yet.) I'm not sure I really have a good handle on Sanguinius's personality, but the best take on him is probably in Fear to Tread. he is charismatic and easygoing, surprisingly down to earth for his outer appearance. The sort of person one would naturally defer to, which is why he was Emperor of Secundus. The other side.... puts Angron to shame for sheer menace, but its controlled. Wrath incarnate, he gives off a pyschic aura of power that was enough to scare the shit out of Kurze and Guilliman at the same time.