Ciaphas Cain has ADHD. It's obvious.
Not that anyone's anything is diagnosed in the grimdark universe. But consider the following.
Lower than average performance in school aside from sports, but his major memories from school he reports are largely social. He may be either holding back recounting ways he struggled, OR his long-term memory favors his strengths. This seems likely to me because he repeatedly recounts situational, sensory, social memories of early life on a hive world that may have been so vivid that they survived whatever mindwiping technology ('Dirus?') that is inflicted on students at a Schola Progenium.
Cain struggles with organization and very often comments that he leaves administrative tasks to Jurgen. He mentions that he loses interest in certain meetings, avoids church functions, and often only skims mission briefings (but seems to have gathered information on his own before many assignments anyway!)
Despite this, he has a ridiculously good attention & memory for names in specific. He recalls names of people he only met for a day over a century after the fact and while he might be looking up duty rosters to re-identify some of these people, others are civilians and would not have been recorded that way. If Cain has the opportunity he asks for names almost immediately when meeting someone, no matter how desperate or incidental the encounter, and reports candid disappointment when he failed to learn/remember a name.
Cain is easily distracted by small things others aren't paying attention to, and this has saved his life and the lives of others more than once.
Despite OR because of this, Cain has very specific attention to details and especially details about people such as how attentive others are around him. He is constantly monitoring other people in his surroundings and becomes more anxious when alone. This is true even outside of dangerous situations.
Cain is cautious of his own impulsivity in a way that is unusual for a neurotypical person, sometimes breaking narrative to explain his own decisions. Sometimes he's right to, but other times he only believes his decision was impulsive. He sometimes confuses his concern about others for impulsivity too, as if he groups multiple kinds of risk into one category 'to avoid.'
He has a preferred environment (tunnels, closed surroundings) that impacts his attention/focus in a profound way.
Many of his lucky dodges occur when his attention is on something an assailant doesn't predict, and this can happen even when the opponent has actual psychic powers.
Cain will get a feeling something is wrong when many others don't, but he isn't psychic. Because everyone has some intuition, we can't assume he just has 'more,' especially not in this setting. It's more likely that his selective attention is actually worse than that of most others around him, which allows him to have intuition about things others don't realize they're ignoring.
Tanna is obviously a reference to Chifir, which is an extremely strong black tea. Among other things, it has a very high caffeine content and Jurgen is pretty much constantly offering it to Cain in the books. Cain doesn't report 'normal' stimulant side effects; the drink relaxes him. Many people with ADHD self-medicate with caffeine as many stimulants affect ADHD brains differently.
Cain's social skills are overdeveloped in a way that suggests masking in excess of his impostor syndrome. He strategizes socially and recounts trying to deliberately control of how he comes across during situations that others would find straightforward. When he claims he did something as a 'practiced dissembler,' he is remembering he had to apply skill-based effort.
We can know Cain is masking and not simply 'manipulative' because he doesn't rely on the direct social benefits of intentional manipulation and instead misinterprets his own attempts to protect himself. Beyond just hiding unflattering emotional states, he will say he routinely engages in 'the modest hero act,' but every time he's only sharing credit as deserved. It's the disproportionate and often unfair positive response to his honesty, one that frames him as a sole heroic individual, that causes Cain to believe he is deceptive in these situations. Feeling misunderstood when communicating clearly and directly is also a common experience among people with ADHD by the way.
Cain's reputation assists his masking which is part of why he is so keen to maintain it. While Cain is considered a hero, he is completely forgiven for having an ADHD communication style. He inserts himself into any conversation including those of very powerful people. He's also unable to resist interacting with those around him, often in ways his job would normally preclude. Cain is a chatterbox among Commissars!
Amberley is protecting us from how bad his organization is, and potentially his ability to ramble by compiling his memoirs into linear volumes with chapters, which she complains about at the start of every single book in case it's a reader's first.
But Cain's imagery is evocative for a private memoir he didn't expect anyone to read, which suggests he naturally wants to communicate this information. ADHD people sometimes struggle to determine what details or context others want to hear. Cain may believe he's writing down something 'candid' without realizing he's describing more than others would.
It's likely that Ciaphas Cain, Hero of the Imperium has combo ADHD. But because the areas he has excessive function in (names, social & situational awareness?) are so successful in his position, and he has the most attentive and doggedly thorough aide in the galaxy, nobody notices an 'issue.' Even Cain thinks that his own behavior is just part of some kind of secret mediocrity.
