Professor Lupin
Background
Remus "Moony" John Lupin is the professor for Defense Against the Dark Arts at Hogwarts School of Witchcraft and Wizardry. He is in a relationship with Nymphadora Tonks. He is a werewolf.
During the events of Harry Potter and the Prisoner of Azkaban (2004) played by David Thewlis
Harry Potter, Hermione Granger and Ron Weasley go into a train car where Professor Lupin is already inside, asleep. As the train is traveling to Hogwarts, the train comes to a stop and the lights go out. Ron sees something moving outside and a group of Dementors come on board. One of them opens the door to their train car and starts to suck the joy from Harry's spirit. Lupin gets up and casts a spell, forcing the Dementor to retreat. Lupin tells Harry the Dementors were looking for Sirius Black. They arrive at school where Wizard leads the choir in a rendition of Double Double Toil and Trouble from Macbeth. Before their feast, Albus Dumbledore introduces the students to Lupin. During class, Lupin shows the class a wardrobe with a Boggart inside of it. He teaches them, a way to repel a Boggart is to cast the spell Riddikulus along with laughter. The students, including Girl 2 then practice chanting the spell. Neville Longbottom is chosen to go first, and then the class continues with Ron going next, followed by Parvati Patil and then Harry. When it is Harry's turn, the Boggart turns into a Dementor and Lupin is forced to return the Boggart to the wardrobe. Harry talks with Lupin about the Boggart and what happened on the train and Lupin tells Harry about James Potter and Lily Potter.
Later, Harry talks to Lupin again and Lupin explains why the Dementors target him more than others. Lupin then agrees to teach him a spell to combat the Dementors. He teaches Harry the Patronus Charm. Harry fails at first, but succeeds when he changes what he is thinking about when he casts the spell. Lupin walks up while Harry is being questioned by Professor Snape and takes the Marauder's Map from Snape. Lupin chastises Harry for not handing the map over and before leaving Harry tells Lupin he saw Peter Pettigrew's name on the map. At the Shrieking Shack, Harry threatens to kill Sirius, but is disarmed when Lupin bursts into the room. Lupin and Sirius hug and Hermione thinks Lupin has betrayed Harry. Hermione tells Lupin she knows he is a werewolf. Lupin tells Harry it wasn't Sirius that betrayed Lily and James, but Peter. Snape runs into the room and disarms Sirius. Harry uses his wand to knock Snape unconscious. Scabbers tries to run away and is transformed back into Peter and is caught. As Sirius and Lupin are about to kill Peter, Harry stops them and says Peter needs to be taken to the castle and given to the authorities. They all exit the cavern under the Whomping Willow and Hermione yells out when the full moon starts to peak over the horizon. Lupin starts to change into a werewolf and Peter manages to escape. Sirius yells at Harry, Ron and Hermione to run. Snape walks out of the tunnel and is swiped at by Lupin who has completely transformed. Sirius and Lupin fight and Harry throws a rock at Lupin distracting him. Something from far away howls and Lupin runs towards the howl. As Lupin is about to attack future Harry and Hermione, Buckbeak saves them and Lupin runs off. Later, Harry walks into Lupin's office and Lupin tells him he saw him coming and points at the Marauder's Map. Lupin tells Harry he has resigned as a teacher due to someone telling others that Lupin is a werewolf. Lupin returns the Marauder's Map to Harry and then leaves.
During the events of Harry Potter and the Order of the Phoenix (2007) played by David Thewlis and James Utechin
Harry, Nymphadora Tonks, Alastor "Mad-Eye" Moody, Kingsley Shacklebolt, Elphias Doge, and Emmeline Vance go to Grimmauld Place where Sirius is having a meeting with Mr Weasley, Lupin, Professor McGonagall, and Mrs Weasley. The meeting ends and Harry says hello to everyone and Sirius gives him a hug. Kingsley hands Harry a newspaper where Fudge is accusing him and Dumbledore of lying about Voldemort. Sirius tells Harry that Voldemort has been trying to recruit followers and that Voldemort is trying to get something he didn't have last time. Later, Harry and Hermione meet up with Ron, Ginny Weasley, Neville, and Luna Lovegood and they go to the Department of Mysteries. They are surrounded by Death Eaters, including Lucius Malfoy, who reveals to Harry that Voldemort tricked him and Sirius was not in danger. They demand the prophecy and a running battle starts. Harry and the others escape to the room of the Veil where they are surrounded by the Death Eaters and his friends are captured. The Order of the Phoenix arrive and another battle starts. The prophecy is destroyed when Lucius drops it. Bellatrix Lestrange casts the Avada Kedavra spell against Sirius, hurtling him towards the Veil, which absorbs him.
During the events of Harry Potter and the Half-Blood Prince (2009) played by David Thewlis
During holiday break, Harry tells Lupin, Nymphadora and Mr Weasley about what he overheard between Snape and Draco Malfoy. Mr Weasley tells Harry that the members of the Order of the Phoenix have been under surveillance and tells him about the Vanishing Cabinet. The Weasley house is attacked by Death Eaters including Bellatrix and Fenrir Greyback.