Without thinking, Harry casts a Patronus in their direction, and he catches the Snitch easily, winning the game. Only afterwards does he discover that the Dementors were, in fact, several Slytherins, including Malfoy, dressed in cloaks with the intention of unseating Harry.
Every time Harry is confronted with a Dementor, he hears the screams of his mother as she is murdered by Lord Voldemort. A horrific memory to hear... He hears this moment because Dementors feed upon happiness and cause depression and despair in anyone near them. They make people relive their worst memories.
Harry and Hermione dart after them and watch as the dog pulls Ron down inside the Whomping Willow, breaking his leg in the process. They follow him, but only after Crookshanks surprises them by pressing a hidden knot on the tree, causing it to quiet its branches.
It's not exactly clear what would have happened if a Dementor had succeeded in kissing Harry Potter, because Harry had a fragment of Voldemort's soul in his body in addition to his own. It is possible that the Dementor would have ingested both Harry's soul and also the fragment of Voldemort's soul.
A theory posted on Reddit suggests Dementors were drawn to Harry because they could sense the piece of Voldemort's soul living in him. Harry is an “accidental horcrux”, created when the killing curse casted by Voldemort backfired, fragmented his soul, and sent a piece of it to Harry's body.
The Dementors made his inner feelings of hurt and abandonment come to life, along with bringing back his worst memories. As mentioned by Remus Lupin, Harry's true fear was fear itself, which the Dementors projected. The creatures forced him to feel the defeated mentality inside of him that he generally pushed down.
Gryffindors are brave. The Dementors pull out the worst memories you have. Harry's worst memories involve his mother's death. Voldemort is just a wizard (okay, a very powerful evil wizard).
J.K. Rowling told us, 'the mood-enhancing properties of chocolate are well known in both the Muggle and wizard worlds. Chocolate is the perfect antidote for anyone who has been overcome in the presence of Dementors, which suck hope and happiness out of their surroundings.
The Dementor's Kiss has only been used as punishment in rare cases. Sirius Black was supposed to receive the Dementor's Kiss in 1994 after his escape from Azkaban, but he narrowly escaped from Hogwarts School of Witchcraft and Wizardry before the punishment could be administered.
Speaking of joy-thieves, she might have preferred to cloak herself in pink and fill her office with fluffy kittens, but Dolores Jane Umbridge was every bit as dark as the villainous figures she liked to surround herself with. Indeed, she was the one who sent Dementors to attack Harry in Little Whinging.
Snape, determined to get them in trouble, followed Sirius' directions. When James learned what Sirius did, he stopped Snape before he reached the Shrieking Shack, saving his life. Snape, however, had seen Lupin in werewolf form and was sworn to secrecy by Dumbledore.
It was Crookshanks who helped Sirius gift Harry his Firebolt. Sirius tasked the cat with taking the broomstick order to the Owl Office in Harry Potter and the Prisoner of Azkaban.
Dobby's injuries were too severe for him to be saved. Magic has its limits. Also, Hermione was not in great shape herself, having been tortured by Bellatrix. She also lacked the advanced medical training, though it really wouldn't have mattered if she were trained as Dobby was beyond help.
It's revealed later that Harry heard his mother screaming, as he was reliving his worst memory: the moment his mother died at Voldemort's hand.
It states that they hear it inside their heads (as opposed to “outside”, aka by their ears), so a Sonorous is out. I think it is an extension of a form of Legimancy, which causes his voice to go directly into their minds. Perhaps, and probably, using dark magic.
Lily Potter
Harry's mother who sacrificed herself to save Harry from Voldemort; Harry can hear her screams when Dementors are near.
Later on in life Marlene becomes the wife to Pure-Blood Wizard Sirius Black and the mother to their children Half-Bloods Aries, and Artemis Black. She is the sister-in-law to Pure-Blood Wizard Regulus Black, Muggle-born witch Vivienne Price and aunt to Half-Blood Renee Black.
The story of Sirius Black and Marlene McKinnon first began in their sixth year at Hogwarts. She was new and he was more than happy to make her feel welcome. His pure-blood status and her father being a muggle made no difference to either of them, even in a time of great prejudice.
A Dementor attempting a kiss on Sirius is witnessed in the film adaptation of Harry Potter and the Prisoner of Azkaban, in which his soul is being sucked out his mouth in the form of a small, glowing, white dot, but the Dementors are fought off by Harry's Patronus before they can finish this.
This is because werewolf-inflicted injuries are cursed. It's possible that Lupin received these scars from Greyback or from other werewolves during his transformation. Additionally, the scars could have been self-inflicted. After all, Lupin was deeply unhappy about his status.
This time, he hears his dad when the Dementor-boggart approaches. Lupin pauses at this and reveals that he knew James in school. But he quickly changes the subject and has Harry try one more time. Harry uses the memory of when he found out he was a wizard and could leave the Dursleys.
Lupin carries chocolate with him on the Hogwarts Express (PA5), which seems to suggest that he expected a problem with the Dementors, or perhaps that any Defense Against the Dark Arts specialist carries chocolate as a matter of course.
In Harry Potter, Riddikulus is a spell used against Boggarts. For the magic to work, you have to think of something funny while casting it. If it works, it makes the boggart take the shape of whatever you were thinking of and because Boggarts feed on fear, they're defeated by laughter.
Voldemort's greatest fear is death and as a result a boggart takes the form of his own corpse. It is unsurprising that this is Voldemort's biggest fear. After all, he did split his soul up encasing it in various horcruxes just so to keep himself from dying.
Dumbledore - Ariana Dumbledore
He fears the lifeless body of Ariana Dumbledore, his sister, with who he associates his biggest regrets.