As can be clearly seen below, he rides a GODDAMN FUCKING BEAR into battle, 'nuff said. Ok fine, we'll also say that he met this bear for an initiation ritual as a priest into the Cult of Ursun (where he had to go into a forest and tame a bear). The tale goes that he tried wrestling it into submission after meeting it but neither could overpower the other and their mutual blood loss attracted a pack of wolves which tried going for the bear first. Boris defended it and was driven unconscious from his ensuing injury. The bear returned the favor and unwaveringly kept him safe (even from Kislevite help looking for their ruler) until Boris woke up, with the two of them having put down over two dozen wolves together. Thereafter, the bear was named Urskin and was ever loyal to him, like an ursine-Deathclaw. A "Bearclaw", if you will.
Boris became the Tzar of Kislev in 2492 IC (968 of the Kislevite calendar) after his father Vladamir got shanked to death by Goblins. Boris quickly realised the the country he had inherited had never really recovered from Asavar Kuls invasion, nearly 200 year prior, and so pissed off almost every Boyar and noble in the county as he nearly bankrupt Kislev training and reorganising Kislev military, rebuilding defences, roads and bridges and hiring engineers from the empire to build canons and black powder weapons. He then set out on multiple expeditions to reclaim and consolidate land that had been lost in the Great War and from other forces in the time since. Finally he reinstated the worship of Ursun as Kislevs main religion, after Ulric and other shitty southern gods had seen an increase in worship, which reinvigorated Kislevs sense of unity and sense of purpose. All of this pretty much pull Kislev back from the brink of collapse and reinstated it as a proper major power in the Old World with an effective and up to date fighting force.
Sadly Boris met his end in 2517 IC while leading an army into Troll Country, when they came upon a Kurgan warband pushing south towards Kislev. The forces clashed and Boris charged into the centre of the Chaos horde, but was cut off by the Kurgans from the rest of the army. Realising that he was separated, he fell upon the classic Russian philosophy of "There Is No Land Beyond the Volga" and he and Urskin made a final stand that would make Boromir proud. Boris and Urskin carved their way through dozens of enemies, taking numerous wounds that should have dropped the bugger 10 times over but he held on through sheer force of will, until Urskin was able to break through the chaos hordes and return Boris to the Kislevite army. Boris finally succumbed to his wounds after directing the battle and securing a victory against the chaos forces, falling from the back of Urskin, dead. Seeing his fallen master and friend, Urskin went berserk and charged off into the north and according to legend he went to slaughter the north men responsible for Boris's death.
In the End times ot is established that the Kurgan warleader was punished by Archaon for losing the battle but spared death for killing Boris. His punishment? Being stuck leading an army of beastmen (which is a punishment if you consider the relative power of their army book) with Ungrol Four Horn as a subordinate.
Urskin is still hunting Boris' killer and makes a surprise appearance at the end of 'of Ice and Snow'. He seems to have grown a fair bit, described as 'The biggest creature any if them had ever scene' two lines after the same group were all looking at a cygor. That's a very big bear. Whether this causes the Kurgan to now be imagined as warhammer's John Candy is up to the reader.
Boris is a pretty strong beat stick character and is "RELATIVELY" reasonably priced for a named character at 375 points (285 for Boris and 90 for Urskin). He has some pretty great stats which allow him to stand toe to toe with most armies kill lords, and a reasonable chance to give Chaos Lords a good kicking. His special rule Kin of the Bear God means he can give Kossars a pseudo Mark of Chaos for 30 points per unit, which gives them +1 attack so it's best used on on big units of kossars to get the most out of it. The Armour of Ursun is a set of full plate, which combined with his shield gives him a 3+ armour save (2+ when he's mounted), and has the extra bonus of every time he is hit by an attack in combat he may throw another one back after the attacks have been resolved (unless he's been killed by them), which is pretty good but the main attraction is The Shard Blade, which increases his Strength by 2 to a unholy S7. Yes, his attacks are as strong as a Bloodthirsters (or stronger depending on the edition). The Shard Blade also forces the opponent to take a toughness test which causes another wound if failed, with no armour or ward saves allowed. All of this, plus the extra attacks from Urskin means he can spit out a disgusting amount of damage in the correct circumstances.
Speaking of Urskin, the big bear is a bit of beat stick all in his own right. He's S6, with 4 attacks so he's quite capable at mauling most infantry he runs into. He has a 5+ armour save from his thick coat and causes Fear from being a big fuck off bear. In addition if a unit he's in combat with breaks and falls back, he MUST pursue it regardless of if Boris is alive or not, so he can cause himself and Boris to be singled out in some situations.