Cato Sicarius
Cato Sicarius is (was?) the Brother-Captain of the Ultramarines' Second Company. Among his titles are Master of the Watch, Knight Champion of Macragge, Grand Duke of Talassar, and High Shiggydiggytee of who gives a damn.
Hailing from the planet Talassar, Sicarius was Veteran Sergeant of the Second Company under Captain Severus Agemman by the time Hive Fleet Behemoth arrived to nom on Ultramar. After the First Company was horrendously nommed heroically killed to a man during the Battle for Macragge, Agemman was promoted to First Captain and charged with filling up the First Company, with Sicarius replacing him as Captain of the Second Company. In the following two hundred and fifty some odd years, Sicarius has garnered a reputation for heroism and valiant actions surpassed only by a few others. Many within the Chapter believe that when Marneus Calgar dies, it will be Sicarius who succeeds him as Chapter Master, even though Agemman (being First Captain) is the designated successor.
He's one of the many fluff-based complaints against Matt Ward, due to rather unbelievable feats such as using a company armed only with combat knives to hold off 10,000 orks, or somehow repelling one million Tau army with only thousand marines, which is even more unbelievable. You know, when you get outnumbered 1000 to 1, no matter how superhuman your soldiers are, they have no chances, even against grots, not to mention tough as nails angry warmonger barbarians or high-tech space communists with an unholy amount of firepower. In the end, of course, it turned out that Shadowsun just manipulated Cato to keep the Marines together, rather than separating them to support Imperial Guard as spearhead. So while Sicarius and his ragtag band was busy liberating three (and a half!) planets, the Tau Empire conquered three dozen more, effortlessly smashing through Planetary Defence Forces and Imperial Guard.
To be fair, he was completely outclassed by a no-name Necron Lord The Voidbringer at Damnos (who was going nuts via the Destroyer madness).
Nick Kyme and Graham McNeill, in the Ultramarines books they've written, have played with Sicarius' character, showing his desire for glory putting the Second Company in extreme danger, and having several of his own sergeants suspicious about his motives. Nick Kyme's Space Marine Battles novel about the fall of Damnos shows the real reason the Ultramarines lost: because Sicarius insisted on winning an actual victory instead of just blowing the planet to kingdom come, which Varro Tigurius ultimately did after Sicarius had his ass handed to him (Which didn't work at all since Damnos became a major Tomb World). That said his way of thinking wasn't wrong the Real Problem was Necrons outnumbers the Ultramarines 500 to one, and unlike other races they get back up, the 2nd was mostly worn down by sheer attrition. Any other force Damnos could have been saved. In Graham McNeill's Ultramarines series, Sicarius acts as self-appointed chief prosecutor (well its one of his roles at Captain of the 2nd) when Uriel Ventris and Pasanius Lysane are put on trial for violating the Codex Astartes, having such a smug attitude toward his fellow Captain that even the Eldar would think he was being rude. While there is no denying he's ambitious, Sicarius takes extreme offense at insinuations that he's gunning for Chapter Master. Which means you can just rationalize Ward's fluff as just propaganda (in fact MOST of the codex reads like what Ultramar believes their chapter is), this wouldn't be the first time 40k had an asshole glory hound painted as the greatest hero ever.
He has something of a canon conflict, as Captain Titus is Captain of the Second Company in Warhammer 40,000: Space Marine. According to the developers, Space Marine takes place in an alternate [read: better] universe where Titus was promoted instead of Sicarius (although they probably still know each other as Sicarius will most likely still be the company Champion). Though Titus might have just as well been one of the previous Captains of the 2nd Company before Severus Agemman, or it takes place sometime in early M42 and Sicarius has died and been replaced with someone much more competent, given there's no set date for the events of Space Marine. Or most likely Relic just went with the default Ultramarine colors and didn't care about the company colors.
Most Ultramarine armies will have his heraldry, due to it being more or less the default.
He also has at least three canon models of him.
Oh yeah he killed a Transcendent C'Tan.