Severus Agemman is Brother-Captain of the Ultramarines First Company. He used to be Captain of the Second Company, with Sicarius (who back then was just a Veteran Sergeant) as his second-in-command. Then the First Tyrannic War happened, and the entire First Company got itself omnomnom'ed by a bunch of angry bugs. Once the tyranids were finally removed, Agemman was promoted to Captain of the First Company (to replace the now dead Saul Invictus), with Cato Sicarius getting his place at the head of Second Company. The problem was that, for a time, Agemman was literally captain of nothing: the First company had fallen to the last man while defending Macragge. So, while Sicarius and the Second Company (who was largely intact) went to get shit done with bolters blazing, Agemman was left with the (rather thankless) job of rebuilding his Company. To compound the damage, Chaplain Cassius has, in a moment of lucidity and ignoring of the Codex Astartes (something extremely rare amongst Ultramarines), created his very own unit of veterans. Cassius' (correct) reasoning was that a new unknown enemy needed new tactics to be faced and overcome, but in the process of raising the Tyrannic War Veterans he gathered many of the relics from the fallen First Company and Space Marines who would otherwise have become First Company. Hence, with his former second-in-command busy doing the job he cannot do due to lack of manpower and his Chapter's most senior Chaplain draining said manpower away, Agemman's job has been curtailed and his influence has been significantly reduced.
He isn't mentioned much in the fluff, beyond a few passing mentions on how he's been getting overshadowed a lot by Sicarius. Imagine having your second-in-command rise to your position and then do everything you ever did a hundred times better then you ever did, and no one ever shutting the fuck up about it: that's probably what its like to be Agemman. And it's only going downhill for the poor man: as Captain of the First Company, he is nominally Marneus Calgar's appointed heir to the rank of Chapter Master. But in spite of this, most of the Chapter believes it will be Sicarius who will replace Calgar when he dies, and not Agemman. And it isn't just idle chatter, as the possibility of Sicarius being chosen over him is actually stated to be a possibility. It is mentioned in one of the short stories that Varro Tigurius has advised Calgar that ever since the Bloodborn invasion, Agemman has been less than reliable and openly suggested to the Calgar that he be replaced as First Captain; either by Sicarus or Uriel Ventris.
Exactly how Agemman feels about all this is unknown, but between trying to rebuild a company that takes forever just to reinforce, while the head Chaplain undermines him, his former second is busy being better than him and could get promoted above him, WHILE the chief librarian not only criticizes him but openly campaigns for him to be demoted, he's probably a little bit mad. In all honesty, though, getting outdone isn't his fault since he's 'only' a regular character trying to compete with one of the beast's magical snowflakes.
He has two canon models. The old Space Marine Terminator Captain, and the new one in the Strike Force Ultra box.
It does seem that despite everything, Agemman's laborious efforts payed off and got him some recognition in recent fluff. The First Company was crucial in defeating the Chaos forces trying to prevent the resurrection of Roboute Guilliman, and after the Indomitus Crusade Agemman was appointed to the rank of Tetrarch of Konor by his Primarch (seems Tigurius was wrong about his reliability).
Due to the invasion of Konor Agemman was first in line against the forces of Nurgle threatening Ultramar. Despite the ferocious campaign waged by Mortarion, the Ultramarines ultimately succeeded in defending the vital system. With the end of the Plague Wars and the Death Guard leaving Ultramar, Agemman now has a cushy job as the Tetrarch of the northern reaches of Ultramar's 500 worlds.