Right, there is only one person in question and this is Remus Lupin. Finally, Dumbledore found Hogwarts someone who is capable, no braggart, serious and amiable. There is only one minor drawback: Remus Lupin is a werewolf. 😉
Already in the beginning he comes to Harry’s help when dementors patrolling the Hogwart’s train cause Harry to faint. Lupin fights them off with a Patronus charm and gives Harry chocolate afterwards, advising him ‚Eat it. It will help‘ (p. 67) and when Harry is reluctant ‚I haven’t poisoned that chocolate, you know…‘ (p. 68). This first prove of Lupin’s competence is later regarded with a great deal of favour by the Hogwart’s matron Madam Pomfrey: ‚Did he, now?‘ [–> gave Harry some chocolate] said Madam Pomfrey approvingly. ‚So we’ve finally got a Defence Against the Dark Arts teacher who knows his remedies.‘ (p. 70). – Exactly!
Besides, I was so happy – finally, there is the proof that chocolate helps in situations of stress and unhappiness.
At Hogwarts, Dementors are regular guests now for the ongoing school year, quite to Dumbledore’s dislike. Dementors are draining all positive feelings from everyone they turn their attention to, so that these persons feel like they could never be happy again. Their last weapon is the Dementors‘ Kiss which condemns a person to live without his soul which the Dementor has sucked out of him. Cheery, isn’t it?
The Dementors at Hogwarts are looking for the escaped fugitive Sirius Black, Harry’s godfather who is falsely believed to be a mass murderer and the betrayer of the whereabouts of Lily and James Potter so that Voldemort could kill them when Harry was still a baby. Harry in particular is hit by the effects of Dementors which is why Remus Lupin teaches him to create a Patronus for himself. Harry’s Patronus significantly turns out to be a stag.
Sirius and Remus were Harry’s father James‘ best friends at Hogwarts when he himself was a student there. Sirius is the younger son of a famous family of wizards, the Blacks, and he was the first family member who ended up in Gryffindor instead of Slitherin. He left home when he was still a teenager to live with the Potters instead to distance himself from his rather Dark Arts attracted family.
Remus was brought to Hogwarts by Albus Dumbledore although he was already known to be a werewolf when he entered school. Dumbledore however protected his secret and made sure that he wouldn’t attack people when once a month he changed his appearance with the full moon. The secret however didn’t last long regarding Remus‘ friends Sirius and James … and back then also Wormtail 👿 who learned to become animagi – wizards who can change themselves into animals like Professor McGonagall can transfigure into a cat. In their outer form as animals (Sirius could change into a large black dog, James into a stag and Wormtail into a rat – Ron’s cat Scabbers, actually) they could keep Remus company without being in danger themselves.
In the end Harry and his friends discover that with Sirius, the wrong person was committed for the Potter’s betrayal and the deaths of twelve Muggles. As a matter of fact it was Peter Pettigrew alias Scabbers alias Wormtail who did it and framed Sirius afterwards for his crimes. But instead of the combined efforts of Sirius and Remus to detain Wormtail (Severus Snape isn’t really helpful in that scene as he is more interested in his revenge towards his hated schoolmates Sirius and Remus), Pettigrew gets away by changing into a rat and disappearing out of sight.
Things in The Prisoner of Azkaban nevertheless turn out rather all right because with the knowledge and help of Albus Dumbledore and his plan carried out cleverly by Harry and Hermione, Sirius manages to escape with the other unlucky fellow, the Hippogriff Buckbeak, who faces an undeserved death sentence. Sad is only that Remus Lupin decides to resign as a teacher although Dumbledore tries to make him stay. Lupin, however, sees already enough difficulties arise for Dumbledore and the school without a werewolf hanging around that is accused for allying himself with a not yet amnestied criminal, Sirius Black.
Finally, here is one of my favourite quotes of the third book. I still remembered it from my first reading and now it will presumably stay in my mind forever because of its indignant humour. When Harry enters Flourish & Blotts, the bookshop on Diagon Alley, the shopkeeper is positively relieved when he learns that Harry already has got his copy of The Monster Book of Monsters which had bitten him already three times that day: “I thought we’d seen the worst when we bought two hundred copies of The Invisible Book of Invisibility – cost a fortune and we never found them…” (p. 53).