Captain Kayvaan Shrike (AKA Captain Beakie/Beakie McAwesome or any other variation of such) is the shadow captain and a major member of the Raven Guard. He is fucking badass and wears the lightning claws once owned by the Raven Guard Primarch, Corax. He also has a supremely epic Beakie helmet. He's also a sentimental old sod who will stay behind after the rest of the company has pushed off to deliver righteous asskicking on other foul xenos and instead spend his time rescuing pockets of surviving civilians and soldiers and killing off any left-over resistance to the Imperials. Making him a goddamn PSEUDO-HERETIC for valuing human life above killing the Emprah's enemies, but that is the day-to-day in the grimdark future of the 41st millenium.
Back in 4th Edition, he even had a rule to lead a unique custom unit called Shrike's Wing, which were Lightning Claw wielding Veteran Assault Marines that Shrike personally trained. Pity they aren't available anymore, though you can take Vanguards for the Shadow Captain and call them Shrike's Wing.
He's also fucking awesome because he's named after one of the most hardcore birds in existence, the shrike, who are know for impaling their prey on brambles instead of outright killing them.