Obviously I can't speak for Desdendelle, but if you want an explanation of the problems with your SPaG, here are the mistakes I noticed. I've no idea which (if any) of these are the ones that bothered Des, but it might give you some idea of where to start fixing things.

William's profile

• Like all warlocks from the continuum, he has a mark - a unique body trait, which identifies him as a non-human
  
  Either replace the dash with a comma, or the comma with a dash.


• The only times he is not, is when shocked, thinking deeply, or embarrassed.
  
  Lose the first comma. Also replace "is when" with "are when" because "times" is plural.


• He was born with ivory white skin, but it didn't raise a lot of suspicion - people tend to think he may be of weak condition
  
  Should be "people tended to think he was of weak condition". You might also want to replace "of weak condition" with something less clinical-sounding and more suited to the melodramatic Victorian context, such as "a sickly child".



• At that time, his other warlock mark has manifested and with it - his magic.
  
  Should be "manifested", not "has manifested". Also, there should be a dash before "with it".


• It started small; cutlery moving on the table, plumes of smoke and sparks coming from his fingertips, toys and small objects vanishing and reappearing in different places, etc.
  
  Use a colon to start a list, not a semicolon.
  
  However, you might want to think about using semicolons instead of commas in this list. I know some people don't like semicolons used this way, but at present the list looks like it might be saying the sparks came from his fingertips and also from toys and from the vanishing and reappearing small objects.
  
  The alternative is to break the long list up into smaller sentences: "It started small. Cutlery moved on the table. Plumes of smoke and sparks came from his fingertips. Toys and small objects vanished and reappeared in different places." This has the added advantage that the small sentences help to convey the smallness of how it started. A big big list doesn't look small to the reader.


• the neighbours started gossiping about their family being cursed and condemned
  
  Are you sure "condemned" is the word you want?


• She didn't know the true identity of the child's father but she believed the Lord and His teachings will absolve the boy of all that is wrong with him.
  
  Should be "His teachings would absolve the boy of all that was wrong with him." It's in our past, even if it was in her present or future.


• When William ceased aging, as every warlock does eventually, he decided to travel the continent
  
  Not a SPaG issue, but why only the continent? This was the 1890s, when the British Empire was at its height. Why wouldn't he visit India or Africa as well?


• Because of his demon blood, he is able to generate energy using his own stamina (or life force, if the spell is more powerful) in order to reshape reality. Because of his demon blood he is able to harness energy from his surroundings, and can use this with his own stamina (or life force, if the spell is more powerful) in order to reshape reality.
  
  Duplicate sentence.


• running water, and holy water, works best
  
  Should be "running water and holy water work best".


• and there is only a handful of warlocks who can do it.
  
  Replace "is" with "are" unless you mean that there is literally a hand that is full of warlocks.

VJ's profile

• Venus does not actually hate her name, it's the attention it brings her that's extremely annoying.
  
  Either replace the comma with a semicolon, or start a new sentence.


• She is a very playful individual, and enjoys teasing her friends, such as when mocking her partner about his age, making provocative comments which make some people wonder if she’s related to Luxury, or having frequent "Trivia Wars" with William to determine who knows canon worlds better.
  
  Another long list sentence. Once again it might work better if broken into smaller pieces.


• Later, her love for literature was so strong, she has decided to abandon her college life
  
  Lose the "has".


• This led to a misunderstanding with her parents, which made Venus run away and find shelter in a nearby forest.
  
  Not a SPaG issue, but didn't she have any friends she could stay with?


• At first, VJ was confused and almost fainted when she learned the Organization can visit her favourite stories at any time.
  
  The confusion, the almost-fainting, and the learning were all the first thing that happened? Change the order of the sentence to something like, "VJ was confused at first, but then she almost fainted when she learned the Organization could visit her favourite stories at any time."
  
  Also replace "can" with "could", because what excited her was the fact the Organization already had the ability to do it in her present (i.e. our past). Yes, it also can also do it in our present (i.e. an unspecified point in her future) but this wasn't relevant to her almost fainting.


• Despite the majority of spies hating searching for badfics, reading and classifying them - VJ was having the time of her life.
  
  That dash looks clumsy. So does the use of "hating" in one grammatical context immediately followed by other "-ing" words in another, totally different, grammatical context.
  
  Anyway, think about rewriting the sentence to something like, "Even though the majority of spies hated searching for, reading, and classifying badfics, VJ was having the time of her life."


• She had fun browsing through the Pit and other places, in order to find the abominations of literature. It was also there when, after her first paycheck, she bought a worn-out coat from the General Store.
  
  I'm not sure what "It" refers to in this context. The last single noun mentioned was the Pit. Was the FFN website really in the General Store with her when she bought the coat?


• Since that time, she would wear the coat everywhere she could.
  
  Replace "she would wear" with "she wore" since even though that's our past-stretching-into-our-present and her present-stretching-into-her-future, it still has its roots in our past and her present, so that's what counts. (Sometimes I think English tenses were designed by Dr Dan Streetmentioner!)


• she requested a transfer to the Department of Mary Sues. While working for the DoI, this was the Department she was reporting badfics to the most.
  
  Replace "this" with "that", because it looks like she was reporting badfics to the DoI while working there.

If you want to swear in Permission prompts or missions, fine, but please keep the Board clean. Thanks.

Right, addressing paragraphs individually:

I'm sure Des didn't intend for it to come across as rude, but I can't speak for him. He also didn't learn English as his first language, and I know several other Boarders didn't as well (HG, who is one of the more coveted betas, speaks German as his first language) and their grammar is fine, so... actually, I'm not entirely sure where that comment came from. I doubt it was meant maliciously, though.

And alright, duly noted. It just kind of jumped out at me, like I said, because 'beating the sh*t' is just more commonly used.

Okay, just so you know how things are done in English. ;)

*rereads* Okay, gotcha. The way I interpreted it in the prompt, the agents sent him to Azkaban post-war, but in the post sent him to Azkaban because he was one of the breakouts. ...I should probably go to bed, it's 2:45 in the morning here and my brain is being silly.