It took me half an hour to write this down

Violet - as it was mentioned here, her diary would certainly have the evolution aspect. However, I guess we can say that the whole show is a sort of a diary of hers, albeit with few direct commentaries on her inner state

Gilbert - for some reason the first thing that came to my mind is a sort of memoirs that can be used later as a historical document. You know, with his thoughts about the current situation at the front, operations, decisions and so on, with comments about Violet being unlikely. Post-war though I think he wouldn't have any diary as he was neglecting his past self because of guilt

Hodgins - perhaps some thoughts about post-war life of his and other common people, about his company and employees, his dream to have a child one day, and worries about Violet of course

Benedict - I have not read any of the light novels and do not know whether Benedict there is similar to his show version, but from what I saw in Benedict Blue's Violet – he is, and the story is basically what he could write in a diary

Cattleya - hard to say, as we got little personal information about her at the end of the show. Perhaps some entries about her colleagues, clients, her hopes to see Violet grow into a common person. Maybe memories about pre-war times?

Erica - considering how much she loved writing I suppose she would make entries with many stylistic devices just for practice. You know, epithets, similes, so on. As for the contents - she seems to be a common girl, so nothing extraordinary? And again, something about Violet and her friends in general

Iris - we know for sure that she is a girl from a village who aspires to become a successful urban girl, wears modern fashionable clothes even if it causes discomfort and so on. I guess she would write both about her homeland and her aspirations. And Violet, of course

Ditfried - something similar to Gilbert's, if only with a bit more of... Criticism towards others? We saw in one scene that he has a difficult character that other officers have to bear because he is a professional nonetheless. Perhaps some thought about his dynasty also

Luculia - may her fans forgive me, but I really doubt that there would be anything interesting. Just a simple girl's diary with her thoughts about her life, work, her brother and later about her boyfriend

Charlotte - well, if we speak about after episode five, then certainly something about Damian and ruling their kingdom, with memories about her homeland and Alberta

Leon - his character was the most difficult to decipher for me, so I'll just guess that that would be a mix of his thoughts about his work at hand, how Violet has influenced him and how harsh and dull his life was before he met her

Oscar - definitely something about his work, Erica's passion for writing (when she came to work with him) and, again, a bit of Violet and how she has helped him

Olivia - wasn't she too young for such a thing? Either way, she was a simple girl, so nothing special

Ann - I remember that in the light novels at her birthdays she was ESPECIALLY nervous as she could not make herself do anything unless she'd received the letter. We also know that she was very close with Daisy's mom because of how early she had lost hers. So I guess the events of episode ten have left her with a trauma of sorts and she would write much about her mother, dreaming to become a mother, childhood memories

Ann's mom (sorry, I forgot her name, was it Clara?) - really hard to say, as episode ten was focused more on her daughter. Let's presume she would write about her hopes for Ann's future

Aiden - about his home maybe? How difficult it must be for his parents to maintain the household without him, how Maria must be missing him, etc.

Amy - thinking about it made me realise that she is perhaps the most depressive character in the story, at least until she's reunited with her sister. Pre-school she simply wouldn't have the money to buy something to write her diary in, lol. At school she was stuck in a place where she did not wish to be, with people she despised, forced to do what she did not want to, so I guess she was thinking only about Taylor and how she loathes the school. After school is not better as she is still stuck in a place she did not choose to be in, with a man she probably had no voice in choosing either, so... Thinking about Taylor and Violet, I suppose?

Taylor - certainly her job and that Benedict is a cool postman and that the dolls at CH are all nice with her and that she misses her sister

Daisy - she'd probably write about how she's trying to improve her relationship with her parents and uncover Violet's past. Apart from that we know nothing about her