Cato Sicarius
Cato Sicarius is 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 Leviathan arrived to non on Ultramar. After the First Company was wiped out to a man during the Battle for Macragge, Agemman was promoted to First Captain, and Sicarius replaced 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 over Agemman, who as First Captain is Calgar's designated heir.
He's one of the 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 unholy amount of firepower. On the other hand, he was completely outclassed by a no-name Necron lord. 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. While there is no denying he's ambitious, he takes extreme offense at insinuations that he's gunning for Chapter Master.
Most Ultramarine armies will have his heraldry, due to it being more or less the default.