Scout
Space Marines noob in training. They have not yet gained the required gene-seed organs or status to wear power armour. Usually organized into squads, and are the only marines capable of wielding sniper rifles. Black Templars do not use scout squads, instead mixing their scouts ("initiates") into their normal battle-brother squads. They apparently think this gives the initiates better experience, but it's really just because they make amazing meat shields.
They are lead by a sergeant (usually a member of a tactical squad that chose to impart his wisdom on a younger generation). In the case of the Blud Ravens, they are led by Steve Blum voicing an angsting, ineffectual dork, and he hopes one day to teach all the Blud Ravens to be angsting ineffectual dorks. Among his most important lessons is that, when facing Tyranids, the greatest tactical decision is to give up all hope. Aside from that, go out of the way to point out problems and question every solution as if it was the most inexperienced idea you've ever heard.
Also, if you even say the word "boink" within a hundred feet of an Angry Marine Scout with a Power Bat, he will beat you so hard you won't know which end of your sternum you're looking through.