Tarkus
Tarkus is the manliest motherfucker in the entire Blood Ravens Space Marine chapter. He has been a steadfast warrior whose capacity for kicking ass is only matched by the power of his heart. According to canon, he's been with the chapter since the Kronus campaign, as one of the nameless honor guard sergeants. Noted for being the single most sympathetic character in all of Dawn of War II, as well as being one of the most beloved by /tg/ due to the sheer ass-kickery he provides, Tarkus is one of not only the most useful characters to join your squad in the game, but also one of the most versatile. Whereas that angry motherfucker Avitus is dour and angry, and Cyrus tends to be dark and brooding, Tarkus is the old veteran - the one that has seen it all, endured the crucible of war, and emerged with his humanity tempered. In a universe filled with grimdark, Tarkus is the exception to the rule, pushing forward with genuine concern for his fellow marines and leaving the universe that much brighter with each bolter shell he fires.
Loyal to the end, Tarkus can, like all members of your squad in Chaos Rising, become a filthy heretic, but even then he does not cease his loyalty to the Imperium and the Emperor.
It's fucking inspiring.
Read on, gentle reader, and learn more of this legend.
Overview
Tarkus himself is a Tactical Marine Sergeant in the Blood Ravens 4th Company. He has over 200 years of experience under his belt, and is an expert tactician as well as a rock-solid second-in-command, who serves the Dawn of War II Space Marine Commander as an advisor and confidant. Amongst his fellow marines, Tarkus is renowned for his confidence and tough-as-adamantine resolve. Tarkus' prominent scars and tendency to be hard on himself stem from a personal tragedy: Tarkus was once convinced by an Eldar Ranger to lead his squad straight into an ambush. The Ranger had approached him under the guise of a truce and had proposed they temporarily work together against their common enemy, the Orks. Suffice to say, in the proud tradition of the Eldar possessing the most hubristic fucks in the galaxy (Assuming, of course, that one ignores the Imperium's hubris), the Eldar betrayed the Marines the second the opportunity arose, and Tarkus' squad was cut down piecemeal - Tarkus was the sole survivor of the ambush, and has yet to forgive himself for this. Tarkus harbors a serious grudge against not only the Ranger (who escaped the battle), but against Eldar in general as a result, and his vendetta against them has not faded over the years.
Tarkus' first canon appearance was in Dawn of War: Dark Crusade, where he supposedly was one of the available Sergeants under the command of Captain Davian Thule. He and his squad were commended for their efforts during the assault on the Necron Stronghold on Kronus, where Tarkus' squad held off wave after wave of Necron forces as Captain Davian Thule planted a Plasma Bomb so as to end the Necron threat to Kronus entirely; as a desperate act, Tarkus brought a Destroyer down by shoving a frag grenade into its neck joints, earning yet more scars and the Crux Terminatus.
Tarkus' native world is Calderis, which is one of a handful of recruitment worlds for the Blood Ravens. It's notable also for being a major training site for the Imperial Guard as well, so Tarkus' powerful humanity may well have been hereditary.
Dawn of War
Tarkus is a Sergeant of the Blood Ravens chapter, and he's been in every war they've fought since the first Dawn of War. He's so fucking awesome he (wears a helmet that he stole that Diomedes gifted to him in Retribution) just uses his awesome baldness to deflect bullets in the first two games. Also, he can be exceedingly loyal. Using chaos to fuck chaos up? That takes balls, and to not truly fall to chaos while using it? Cast-Iron Ferric balls, man. Cast-Iron Ferric balls. This occurs during the Dawn of War II games. He is also revealed to be The Ancient, a fairly (oh who the hell am I kidding, EXTREMELY) awesome strong and silent type character. The first time you find out who the Ancient is, you'll weep with the pure awesome baldness. After all: who actually thinks that overcoming daemonic influences and/or possession is a bad thing?
A simple demonstration of his awesomeness occurs if he turns traitor; while all the other traitors, including Cyrus, have no idea where you're going next and just tell their forces to be on alert, Tarkus knows exactly which part of his base is about to get hit.
In Gameplay
In game, Tarkus is probably the second most versatile squad you can field next to Cyrus. His tactical advance makes him invincible, which, coupled with the artillery strike ability, makes him a walking ordnance beacon. Watch as he stands in the middle of an orbital bombardment without so much as a scratch. This ability also completely overshadows the iron halo, since it drains energy over time, not when damage is taken. This lets Tarkus laugh off most everything for extended periods of time on the order of five minutes, rendering him a persistent thorn in the enemy's side. Tactical advance can also be upgraded to increase Tarkus's damage or give a measure of anti-suppression to the rest of your squads.
Since he's hardy as all fuck, Tarkus works perfectly with short-ranged weapons like flamers, meltaguns, and plasma guns. It's relatively easy to have him wade into hordes of enemies to kill linchpin opponents like Tyranid warriors and Nobz, or tie up ranged squads in close combat. His other skills combo perfectly with his durability, letting him draw ALL THE FIRE while the rest of your force gets into position. Tarkus is also a master of grenade warfare, able to throw unlimited grenades, and eventually, throw bunches of grenades.
One more thing of note is that his squad has more meatshields unnamed marines, coming in at three over everyone else's two. These can be upgraded to chapter veterans, giving you two extra cheap plasma guns on the field, and the upgrade falls on the road to super-tactical-advance, meaning that it's a no-brainer not to get it.
He doesn't have the strongest guns, or the most effective explosives, or the best melee, but his Marines are the most vital part of your team. He will not die, he will mark artillery, and he will buff your entire team to unstoppable levels. Simply put, Sergeant Tarkus is the best Blood Raven in Dawn of War II, bar none. Yes, that includes Davian Cool and Gabriel Angelos.
He is also able to affect Tyranids with the taunt ability. This can only mean that Tarkus is so skilled at the art of insults that he draws the attention of the Hive Mind itself.
In Retribution, he somehow manages to become even cheesier as he gains the ability to use any weapon that isn't a shotgun, sniper rifle, or 2-handed melee weapon, make himself invincible whenever he uses any of his abilities, and gains a jump pack too, essentially making him a cross between a Sternguard Veteran, a Vanguard Veteran, and a Devastator Marine. Give him the Shadowscale armor wargear that renders him permanently invisible and chapter rites accessory, and a plasma cannon, plus the final offense ability to call in air-strikes, then watch him demolish everything in his path with impunity.
Canon
Who gives a Damn about "canon"? He's awesome, no two ways about it. And, thankfully, nothing has happened to him like, say, being equipped with a multi-laser biotic arm by C.S.Gotohell.
External Links
Good betrayal, or BEST betrayal? Tarkus explains why he fell to Chaos.