Topics that people may need to be aware of before their characters interact with Bull are: 

Bull's past as a member of his culture’s version of the secret police, prolonged fighting in a chaotic place where he saw a lot of horrible shit, seeing friends die, having PTSD and a mental breakdown, the consideration he gave to ending his life during that period (he ultimately decided against trying, and openly talks about it in canon, when asked), and voluntarily going into government-administered brainwashing/reeducation. He's also hypervigilant, paranoid about whether unfamiliar people are going to put poison into his food or drinks, and casually imagines how to fight and kill people if they become threats, 'in case it comes up'. His fighting style is brutal and bloody and faced with people he can think of as 'the bad guys' he revels in it, so if I write him fighting I'm going to default to reflecting that mindset with no out-and-out criticism of the actions it can lead him to.

Bull usually won’t talk about those parts of him or his past in detail unless those topics come up (for example if he happens to mention Seheron and a character asks him about it, that will probably lead to at least some discussion of those topics). He also might talk or think about liking violence or needing it and seeing himself more as a tool to be put to some use than as a person, and if the topic of the authoritarian government he grew up under comes up, he'll probably defend its policies (possibly even after leaving the Qun, if to a lesser degree).

If you need me to make sure any of these topics don't come up in his action, dialogue, or his internal narration, please let me know! All comments should be screened for this entry, so anything you say should stay private.