Cyrus
Cyrus was a Scout Sergeant of the Blood Ravens, who debuted in Dawn of War II. Surprisingly, he previously served in the Deathwatch, from where he gets good knowledge about various aliens. Much like Avitus is capable of displaying only the emotion of rage, Cyrus provides the squad with a mix of bitching, morbidity, and cynicism, which causes him to play rather badly with the naive Thaddeus, and also causes Tarkus to call him out, forever demonstrating that the tactical marine sergeant is the bringer of all that is good. On the other hand, Cyrus's outlook does play well with Avitus's, who is angry and morbid, instead of just morbid. In short, Cyrus is more or less the direct opposite of most of Steve Blum's other roles. He's also notably hair-etical, like the majority of your strike force, and is distinguished even from Commander Vanilla Ice and Gabe Angelos in having a beard. However, he does call Indrick Boreale incompetent, which means that he can't be all that bad.
Dawn of War II
Cyrus, along with Tarkus and Avitus, accompanies you on the first mission and for the rest of the game. He tends to advocate pragmatism and is the main source of information you have about the Tyranids, having acquired information about them while serving in the deathwatch. Thaddeus doesn't like him very much on account of his morbid whinging, and Tarkus also has some conflicting interactions with him because Cyrus doesn't know when to back off. This is the game where he has his finest moment, highlighting the failure or Indrick Boreale and by extension the eye-bursting fail that was Soulstorm.
Chaos Rising
Cyrus goes Angry Marine on the as of a sorcerer who's stealing his scouts, and gains corruption of not brought along to kick the pansy faggot's ass. Given that it's a bunch of soft targets, with a few campled in cover, against missile-toting plague marines and chaos dreadnoughts, it generally goes about as well as you'd expect, which is to say on any difficulty worse than hard, every single initiate gets totaled. He also gives you intel on the forces of chaos that turns out to be complete bullshit, as Tarkus aptly points out. If he does go traitor, he cites the incompetence of Indrick Boreale, Eliphas the Inheritor, Azariah Kyras and Gabriel Angelos for fucking up the sector and the Blood Ravens in particular, and decides to ally with Ulkair to remake the sector to his liking. This indeed proves that he doesn't know what he's talking about, as disparaging the two best characters of the entire Dawn of War series and a man cunning enough to become a double daemon prince is BALD and FEWLISH. There's also the fact that a scout sergeant tries to take on three terminator squads and a dreadnought, fully expecting to win.
Retribution
Cyrus returns in Retribution as a companion to Apollo Diomedes on the quest to shitkick Azariah Kyras. For once, his bitching is appreciably useful, as it plays some role in helping Diomedes overcome his pride and recognize Kyras as a traitor. It may also have something to do with the fact that he's bitching at Boreale 2.0 instead of an avatar of the player. He's usually the one who does the mission briefings, since he's the only one who can really scout. On the whole, he remains almost exactly like he was previously, and continues to be useful on the field while contributing nothing of value otherwise.
Gameplay
Surprisingly, the amount of irritation Cyrus evokes in most players is directly proportional to the amount of Cheese and explosives-based rape that he brings to your force. Simply put, in both Vanilla DOW II and Chaos Rising, Cyrus is almost always a necessary addition to any force, capable of ravaging just about everything with copious use of demolition charges, remote-detonated bombs, cluster mines, and sniper fire. All of this can be done without breaking infiltration, making him equally capable at reviving fellow squad leaders, killing hordes, and picking off high-value targets. He's also one of the most resource-efficient ways to level enemy bases, solo whole levels, and kill tough vehicles, since refills for his explosives are scattered everywhere on the map. Only missions that dropped you straight into an enemy camp tended to cause any trouble for him, as do auto-detecting bosses like Bonesmasha and the Avatar of Khaine.
While Cyrus was a hefty slice of cheese in Vanilla, Chaos Rising turned him into a colossal, all-annihilating wheel of dairy. With a relatively simple upgrade, explosives become energy-based instead of limited items, meaning that patience is literally the one thing that's required to kill everything, everywhere, ever. As if that was not enough, the upgrade also makes explosives more potent, letting you toss out clusters of mines that one-shot infantry, walkers, and tanks, and let you absolutely rape defense missions by creating minefields to rival Vraks. Most everyone on your force gets something similar, so at least Cyrus is slightly more balanced with his comrades; however, most of his comrades can't pull off their tricks while invisible.
He retains the majority of his skillset in Retribution, the most notable enhancement coming in a further buff to his infiltration skills, wherein he can now regenerate energy while invisible, meaning that he can prance around maps without taking a break to refill his energy reserves. However, since all explosives and equipment are energy-cost instead of limited use now, he has lost the massive advantage he had in previous installments, and ceded the coveted cheese-throne to Eliphas and LORD-Commissar Bern. Furthermore, with the honor guard that replaces him being assault terminators, it's now a valid choice to leave him out of battle.
External Links
Cyrus insults yet ANOTHER Blood Raven far better than him.
Cyrus infiltrates his way to Chaos.
Cyrus takes his dickery to the next level and betray the Blood Ravens.