Old bit of a Pathologic analysis - some info could be wrong, but I haven't finished my playthrough
Daniil Dankovsky’s mentality reflects that of the in-game creators – the children (known as the Powers That Be) that are playing the game as you follow through it – as they themselves process grief. A common mentality that people – commonly found in children or atheists – adopt after discovering death is wanting to be immortal or ‘cure’ death. This is primarily due to fearing that nothing will come after death and not wanting your existence and time to end. This is the mentality that Daniil has and pursues. The clause for Daniil is that he suffers from suicidal ideation, leading to an interesting internal conflict between immortality or experiencing an early death. In Pathologic 3, these two aspects are broken down into Apathy and Mania, with apathy leading to suicide as the reality of his situation becomes too hard on him, and mania leading to a heart attack due to his mind racing with concepts and ideas, going absolutely berserk with irrational thoughts - particularly those that makes him believe that immortality is possible. The two other protagonists also reflect the mentality of those after loss/discovering death.
Artemy is a surgeon and likely became one after/whilst his father died from the plague. It’s quite common for people to pursue a role in the medical field after losing a loved one to a health complication/disease/terminal illness. As well, Clara’s magic healing powers depict both how children might play make-believe to cope (similar to the Powers That Be), but also how people can lean towards the supernatural after the death of a loved one.
Artemy and Clara are not seen as childish for what they represent as one is a grown man who is a qualified surgeon for the townsfolk, and the other is a young girl in a town with a strong hold on the supernatural - as well as the stipulation that she is the sister of the Shabnak-adyr, making the idea that she has these powers believable. Daniil consists to be the only outlier as he is a grown man, but his goal is universally known as being unachievable. Despite being a man of logic, he still follows this path, knowing that it could all be for nothing. In his defence, he does get quite far with his research – finding that resurrection might be possible and immortality could be achievable. Daniil himself reflects on these points, and acknowledges that the specimen presumed to be immortal, may just have extreme vital longevity, yet still sees this as a positive and an imperative aspect towards his research.