In CK2 the mercenary companies are basically all the same other than the troop types they have, maybe they could be given a bit more character, like by be given attributes which can maybe change depending on Company leader and maybe events:
- Loyalty: How willingly they stay with the employeer, low loyalty companies would be risky to hire because they could be bribed to leave/defect while companies with strong loyalty will basically only leave if you can't pay them.
- Discipline: Companies with low discipline will likely plunder the local which may make them and you disliked with companies with high discipline will avoid doing such and thus cause less problems for you.
- Religion could matter for some companies more than others, same for Culture.
- Reputation: Probably a value that can change alot depending on stuff happening with the company but generally the higher the reputation the more sought after the company is by the ai and the more competition there is for a company the higher their cost to employ could be. Reputation could may also increase the possible hire range of the Company as more distant realms get knowledge of it.