In Total War: Warhammer and Total War: Warhammer II, Kislev is treated as just another minor AI-controlled faction in the Empire category, sharing the technology and graphics with it. However, it is a distinct subgroup in the game's files and has some unique commandments included.
Defensive - More concerned with maintaining borders than expanding them. Conquest is not their business.
DistrusttheEmpire - This leader doesn't trust most Imperial leaders, and that has an effect on diplomatic relations.
In Warhammer Fantasy lore, Kislev is strongly based on Russia, Ukraine, Poland and Lithuania. Kislev has a couple of units, the winged lancers and gryphon legion, that is heavily inspired by the famous Polish Winged Hussars. But the most other units, culture and appearance inspired by myths and history of Russia and neighbouring lands had been under Russian patronage. The Kossars form both the backbone and standing army of Kislev, inspired by the Cossacks - Russian and Ukrainian frontier settlers in steppes and foremountains in settles called "stanitsa" and led by atamans. Kislev is led by a Tzar or Tzarina and is a strong ally of The Empire.
Though they have their own category on the Diplomacy screen, they still use the Empire unit roster, albeit limited to tier 1-3 units with limited heroes and hero mount options.