Azrael
Azrael is the secretive Supreme Grand Master (Chapter Master) of the already secretive Dark Angels Chapter of Space Marines.
History
Azrael was recruited from the brutes of Kimmeria, but does not show their clumsy-ass thuggish behaviors. He is best known within his chapter for caring for the banner, the scouring of Truan IX, and kicking the shit out of some techno-revivalist cult on Faze V. After that, he disappeared like Robert Pattinson's balls after Twilight. He must've got shit done over those few decades, because he was promoted to a member of the Deathwing, followed by Captain of the 3rd Company, and finally the Master of the Deathwing.
Azrael became the Chapter Master of the Dark Angels after the previous master's demise, becoming the head of the Inner Circle, the big boy table of the Dark Angels that keeps the secret of the Fallen Angels. When he became Keeper of the Truth he received the traditional Lion's Helm, the master-crafted Sword Of Secrets, and a combi-plasma gun known as the Lion's Wrath.
Tabletop
In-game Azrael is the Dark Angel's ubiquitous "costs as much as a Land Raider" HQ. He isn't all that bad and he gives an invulnerable save to the unit he joins, but he cannot take Terminator armor to thug it up with Terminators (that is Belial's job) or a bike (much less Sammael's LAST IMPERIAL JETBIKE). Overall, not a terrible unit but his place in the army is limited. With the changes to 6th, he is FAR more survivable in combat due to his Artificer Armor, but on the other hand his power sword is now only AP3.