Thaddeus
Overview
A Blood Raven assault marine sergeant, Thaddeus is a confirmed hairetic, almost to the level of Commander Hairgel. He is the first recruit from the hive world Meridian in centuries, having been drawn from the underhive gangs; this may in fact explain his ridiculous hairdo.
Serving in the commander's strike force, he is notably the youngest member, being only 80 or so when the events of Dawn of War II occur. This means that he spends a lot of time discussing what it's like to become one with the chapter, and acts as the naive new guy, in contrast to the cynicism of Cyrus and Avitus. Opinions vary on the quality of his speeches, given that they lack the grimdark of Avitus or the manliness of Tarkus.
Dawn of War II Story
Thaddeus first appears in DOW II, joining your commander early on, although not in the first mission, as Avitus, Tarkus, and Cyrus do. He's notable for butting heads with Avitus over the latter's disdain for civilians and guardsmen, and to some extent annoying Cyrus by ruining the scout sergeant's cynicism. Tarkus does his best to mentor him in the ways of being an Astartes.
Thaddeus remains much the same in Chaos Rising, showing his love for the common man by gaining corruption if not deployed on a mission to save the hab-spires of Meridian. If he ends up betraying your squad, he reveals that he made a bargain with Ulkair to allow the Litany of Fury to triumph against the hive mind, and attempts to "save" you by jumping on top of your heads and inserting a chainsword into your orifices. The former portion of this is almost certainly bullshit, as Jonah Orion, who actually defeated the hive mind, noted that he felt no daemonic presence.
He does not reappear in Retribution. The common assumption is that he was sent alongside Hairgel into exile, or on a crusade of penitence into the Eye of Terror.
Gameplay
Thaddeus is relatively good at melee, and with the right build can be made invulnerable for a while while assault jumping on everything. This naturally becomes even more effective when he is given a good suit of terminator armor and a thunder hammer/storm shield combo, which also aids his staying power. Because Primarch difficulty turns marines into guardsmen, given how everyone in your squad dies the moment an enemy bops them on the head, making melee combat very ill-advised, it is viable to spec Thaddeus for short-ranged combat. By equipping him with grenades or satchel charges and flamers, having him funnel enemy troops into a single area by using his assault jump, and then throwing explosives at them while they're still reeling then cut down the survivors en-masse with your flamer (And you gain another one by getting his veteran upgrade); Thaddeus can be an effective 1-shot ambush unit.
Thaddeus's main drawback is that he is incredibly fragile for a melee unit, and higher difficulties will lead to him getting murdered by just about everything, since he can't tank worth a damn. In addition, he doesn't get the ability to use items for energy in Chaos Rising, which gimps him even more. On the whole, Cyrus, Hairgel, and Davian Thule are far better at dealing with enemies in cover, and the latter two are far more capable in melee as well. Like Avitus, a perfectly adequate unit that is outclassed by cheese.