Character

The way the Thunder Kings work is that every unit is trained to do essentially everything at a fair to middling level of competence, and then to specialize from there. Brennus essentially learns everything he possibly can about a front; usually the Kings get sent to out of the way and difficult, widespread wars, so it is likely that the IA have been fighting there for some time. He (or his Councilmen, the company commanders) spread out their forces so that there's a moderate presence throughout the area they're taking, usually no more than a few thousand in any given battlefield. This avoids a lot of the collateral damage huge numbers of marines cause (which Brennus hates), and helps inspire the Imperial Guard to fight harder. When the Kings conquer an area, they tend to leave a fair amount of humans and oftentimes a small company of marines to help with the transition to Imperial rule, and rebuilding. Brennus feels it is his job as a king who serves a higher lord to make sure that the places he conquers are as easy to deal with for his Emperor as possible, and so he almost never exterminates worlds unless they are xeno worlds. The marines of the Thunder Kings tend to believe that they are the greatest heroes of the human race, and though they expect the best of treatment and greatest respect, are magnanimous as heroes should be.  They lead from the front in almost all cases, actively shield “lesser warriors” such as unaugmented humans from harm, and regularly lead victory celebrations for their human allies. It is not surprising for a member of the legion's Filid to devote many lines of a poem to acts of bravery or valor by the Imperial Army they fight alongside.