Avitus
Avitus was a Space Marine Devastator in the ranks of the Blood Magp- uuh I mean, Blood Ravens and was featured with the rest of the Blood Raven squad in Dawn of War II. Was, because while it isn't directly said, it's heavily implied confirmed that he was the canonical traitor during the events of Chaos Rising. Avitus was well known for his extreme hatred of anything that wasn't a Space Marine (you tryin' to imply something Avitus?). Now, you may think Avitus is your classic SPESS MEHREEN MUST KILL DA HERESIES, but in reality he is a hulking mass of pure emotion. Well emotions, since it's really a mixture of rage, contempt and generally being a sad person. This tends to leave him at odds with Thaddeus, who is idealistic, and Tarkus, who does not like smack talk about other servants of the Emperor.
One thing everyone can agree on is that Avitus should have been an Angry Marine.
History
Avitus, like many of the other Blood Ravens, was a veteran of many battles, but more specifically, the Kronus campaign where the Blood Ravens fought against he Kronus Liberators in the Battle of Victory Bay. The Blood Ravens won, but Avitus was still pretty fucking angry at the guardsmen for killing some of his battle-brothers, calling them traitors and weaklings which seems to have been a foreshadowing of future events and the start of a long history of hilarious outbursts regarding the Imperial Guard. This also wasn't helped by his hometown being oppressed by a corrupt guard regiment, resulting in Avitus projecting his hatred on regiments that were actually decent, like the 85th Vendoland.
Dawn of War II
Avitus's squad is with you from the start of the game, appearing in the first mission while tenderizing some Orks. He's a good source of hilarious quips revolving around the reduction of enemies into meaty salsa, and quickly becomes Cyrus's ally in dispensing a tide of morbidity. Most notably, he foreshadows the fact that Azariah Kyras is a traitor by noting that the chapter has far too many secrets and lies to be fully legitimate.
Chaos Rising
In Chaos Rising, Avitus remains more or less identical to his previous appearance. In the first mission, he gets to accompany Hairgel to kill some traitor guard, which is more or less his wet dream. He generally advocates doing pragmatic things that will up your corruption meter, such as blowing up the gate to Angel Forge. If he turns traitor, Avitus becomes the only heretic to use terminator armor against you, along with a cyclone missile launcher and assault cannon. This still leaves him nowhere near a certain chaos champion or a techmarine in annoying boss fights but he will still wreck your shit if you ever give him the chance to fire any of his ranged weapons at you, especially with his cyclone missile barrage in Primarch difficulty.
During the fight he expresses the nature and the guilt of his heresy in true Avitus fashion: with pure, unbridled rage. With his fall into heresy further breaking whatever little morality he had left, he has come to the conclusion that everything taught to him and everything he knows over the centuries is a lie and that the galaxy is a pointless, nihilistic place that only demands endless killing. Which is quite true. *BLAM* No man died in His service that died in vain!
Retribution
The Ancient reveals that Avitus was the canonical traitor, saying that a friend of his from Kronus was corrupted by Kyras. This automatically discounts everyone but Avitus, since Davian Thule, Martellus, and Cyrus are still loyalists and Thaddeus was recruited after the Kronus Campaign. Furthermore, no one but Avitus is known to be familiar with Tarkus from before the events of Dawn of War II.
Gameplay
In the game, he provides suppression with heavy bolters or high single-target damage with missile launchers and plasma cannons, which is balanced by his squad's relative fragility, necessity of standing still to fire, and complete lack of capability in melee. If you are tactically deficient you can equip him with a bolter, and you will deserve everything that happens to you. He becomes more useful for some playstyles in Chaos Rising, where he makes artillery strikes and cyclone missile barrages energy-costing abilities.
This makes him a well-balanced unit in a force containing Tarkus, Cyrus, and Davian Thule, meaning that he is hopelessly outclassed in damage potential by the latter two and in health by the former. Overall solid, but not necessarily the optimal choice for a Powergamer's squad. Unless you used him with a special heavy bolter which never ever needs to reload.... Throw this with him never having to set up AND causes mini-explosions with each shot in focus fire ( also handy when he gains energy from evey kill he makes) he will rape everything in his arc of fire. Of course, the moment he gains Terminator Armor and a good Assault Cannon, he will cause everything short of heavy vehicles to suffer critical existence failure the moment he uses focus fire.