In _Harry Potter and the Chamber of Secrets_, Harry and Ron find out that the Hogwarts caretaker, Argus Filch, is a Squib.

> "And what on earth's a Squib?" said Harry.  
> To his surprise, Ron stifled a snigger.  
> "Well – it's not funny really – but as it's Filch ..." he said. "A Squib is someone who was born into a wizarding family but hasn't got any magic powers. Kind of the opposite of Muggle-born wizards, but Squibs are quite unusual. If Filch's trying to learn magic from a Kwikspell course, I reckon he must be a Squib. It would explain a lot. Like why he hates students so much." Ron gave a satisfied smile. "He's bitter."  
> <sub>— _Harry Potter and the Chamber of Secrets_ by J.K. Rowling, Chapter 9, "The Writing on the Wall"</sub>

Filch reluctantly talks about it in front of a number of teachers, such as Snape and Dumbledore.

> "[...] He found—in my office—he knows I'm a—I'm a—" Filch's face worked horribly. "He knows I'm a Squib!" he finished.  
> "I never touched Mrs. Norris!" Harry said loudly, uncomfortably aware of everyone looking at him, including all the Lockharts on the walls. "And I don't even know what a Squib is."  
> "Rubbish!" snarled Filch. "He saw my Kwikspell letter!"  
> <sub>— _Harry Potter and the Chamber of Secrets_ by J.K. Rowling, Chapter 9, "The Writing on the Wall"</sub>

Yet Umbridge, [who had a Squib brother herself][umbridge], did not know he was one.

> "Don't stun them, Filch!" shouted Umbridge angrily, for all the world as though it had been his incantation.  
"Right you are, Headmistress!" wheezed Filch, who as a Squib could no more have Stunned the fireworks than swallowed them.  
<sub>— _Harry Potter and the Order of the Phoenix_ by J.K. Rowling, Chapter 28, "Snape's Worst Memory"</sub>

**So who all knew that Filch was a Squib?**

[umbridge]: https://scifi.stackexchange.com/a/71712/19561