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, the Captain of the 5th Company and Master Of The Marches by the time Hive Fleet Behemoth arrived to nom on Ultramar. After the First Company was horrendously nommed to the last 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, and Caito Galenus replacing Sicarius as Captain of the 5th 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 nominally 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. Oh yeah he also killed a Transcendent C'Tan. Without being instantly turned to dust. Though somehow he was completely outclassed by The Undying of Damnos (who was going nuts via the Destroyer madness). (If you must know, he broke his own self imposed rules and used a vortex grenade)
He also has at least three (more like 6, actually) canon models of him (the one not shown here is the Captain form the Assault on Black Reach starter). Meanwhile, Tu'Shan is still waiting for one model and rules, while the Iron Hands are still waiting for special characters.
Most Ultramarine armies will have his heraldry, due to it being more or less the default.
Un-assholing the asshole
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). Gods of Mars also noted this with one character (present during the Damnos Evacuation) saying that meeting Sicarius was the biggest anti-climax of his life because Sicarius gave every impression of feeling like he had lost personally, not the Ultramarines or the planet's population.
That said his way of thinking wasn't wrong, the Real Problem was that the Necrons outnumbered the Ultramarines 500 to one; and unlike other races they get back up, while the 2nd was mostly worn down by sheer attrition. Were any other forces present, Damnos could have been saved. In Graham McNeill's Ultramarines series, Sicarius acts as chief prosecutor (which is, technically, one of his roles as 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, they're just usually in the Imperial Guard.
That said he's been getting some character development turning him into a rather likable guy, namely in Veil of Darkness and Warzone Damnos. Such as the reason for his smugness during Uriel Ventris was because Cato was a long time friend (and Champion) of Captain Idaeus blaming Ventris for his death. On top of that, the typical Ultrasmurf Mary Sueishness aside, Sicarus is the only Ultramarines special character who regularly wears his damn helmet, and that’s gotta be worth brownie points for (un)common sense, at least.
And oh Emperor, that armor looks awesome!
Canon Conflict
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 5th Captain). 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.
On the Tabletop
On the table Sicarius has a bunch of rules aimed at enhancing his force, rather then being a single powerhouse himself. which is one of the few likable things about him. He gives +1 to reserve rolls(as long as he's already on the table), gives one Ultramarine tactical squad Counterattack, Infiltrate, Scout, or Tank Hunters, and lets all Ultramarines use his leadership. He's ok in assault, his sword is only AP3 with the option to trade his attacks for a single one with +2S and Instant Death. He also gets Feel No Pain from his armor.
Ancient Inspiration
In ancient Rome, Sicarius was the word for murderous street thugs, named after their sickle-like bent dagger, and the word carries on as sicario, roughly meaning hired assassin in Portuguese, Italian and less often in Spanish. You could hide it (the dagger, not the thug) under your tunic. You better did, for the law prohibited being armed in the city. Because of this, using a sica was considered having no honor, while gutting your political opponent with, lets say, a gladius was totally okay, especially if you challenge them to get down in the Colosseum with you, while the crowd goes observant cornflakes.
One should wonder, what does this say about our smurf guy here? Considering GW usually picking telling names when speaking Latin? And Cato's visual appearance so clearly being that of a Roman centurion? Those connections make sense, as there are two renowned Roman Republic traditionalist senators (who also served as generals without any formal training) by the name of Cato, Cato the Elder and Cato the Younger respectively great-grandfather and great-grandson respectively, and they generally make the High Lords of Terra look like very nice people, like all ancient Roman politicians we have record of, and were also considered by their people as morally upright and incorruptible since they refused bribes and followed traditional virtues, because that's totally all the credibility you need to be a good person.
Gallery
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The old Space Marine Captain mini from the 4th edition was actually labelled as the model for Captain Sicarius.
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Sicarius's previous mini. Back during his wolverine phase.