Why was Lupin comfortable with saying Voldemort's name?

Question

When Harry and Lupin had a chat in Lupin's office on the first Hogsmeade trip in the Prisoner of Azkaban, Harry asked why Lupin didn't let him have a turn with the Boggart that they had done a few days ago. Lupin's reply was that he was afraid that it might have turn to Voldemort. This surprised Harry, as at the time he and Dumbledore were the only ones who consistently said Voldemort's name out loud.

What made Lupin specifically unfazed by the usage of the name? Was it because Sirius Black was on the loose and (at the time) thought it would make him seem intimidating to him if they reunited?

Answer 1

There is no canonical explanation, so the best you'll get is speculation. Here are a couple possible reasons Lupin says "Voldemort."

1. Lupin grew up with Sirius and James who often encouraged him one way or another to do things that were brave or risky, such as running around with them in his werewolf form or getting into trouble. They certainly played a role in forming his personality and would have influenced him to say "Voldemort."

   Going through old detention records with Snape, Harry repeatedly sees “his father or Sirius's names, usually coupled together in various petty misdeeds, occasionally accompanied by those of Remus Lupin and Peter Pettigrew”. -Harry Potter and the Half-Blood Prince, Chapter 24 (Sectumsempra)

2. Bitten by Greyback, Lupin was directly hurt by the dark arts in a way that others who do not say "Voldemort," such as Molly, Hagrid, or McGonagall, hadn’t experienced. He had a personal reason to want to fight the dark arts. We see the same drive develop in Hermione when she realizes her sense of purpose during the creation of Dumbledore's Army.

   "Harry," she said timidly, "don't you see? This ... this is exactly why we need you.... We need to know what it's r-really like ... facing him ... facing V-Voldemort." It was the first time she had ever said Voldemort's name and it was this, more than anything else, that calmed Harry. -Harry Potter and the Order of the Phoenix, Chapter 15 (The Hogwarts High Inquisitor)

3. Dumbledore majorly improved Lupin's life by letting him into Hogwarts, causing Lupin to hold him in very high esteem. This possibility seems least likely, considering Dumbledore helped Hagrid in almost the same way and Hagrid didn't say "Voldemort." Every character is unique and Lupin's respect for Dumbledore may have encouraged him to be like Dumbledore (i.e., saying "Voldemort") whereas Hagrid might not respond the same. Lupin felt guilty about betraying Dumbledore's gift of acceptance and when Dumbledore told the Order to say "Voldemort," Lupin was not about to do anything again that would disobey his wishes. But this seems like a stretch.

   "I sometimes felt guilty about betraying Dumbledore’s trust, of course he had admitted me to Hogwarts when no other Headmaster would have done so" -Harry Potter and the Prisoner of Azkaban

4. The most likely reason is that Lupin, through his experiences and characteristics, was just the kind of person who would say "Voldemort."

Ultimately, there's no black-and-white, explicit reason Lupin gives for saying "Voldemort."

Answer 2

Dumbledore encouraged his allies to use it.

Dumbledore encouraged those around them to use the Dark Lord’s name. He tells McGonagall that he’d been trying to convince people to call him by his proper name for eleven years.

"My dear Professor, surely a sensible person like yourself can call him by his name? All this 'You-Know-Who' nonsense – for eleven years I have been trying to persuade people to call him by his proper name: Voldemort." Professor McGonagall flinched, but Dumbledore, who was unsticking two sherbet lemons, seemed not to notice. "It all gets so confusing if we keep saying 'You-Know-Who'. I have never seen any reason to be frightened of saying Voldemort’s name."

"I know you haven’t," said Professor McGonagall, sounding half-exasperated, half-admiring. "But you’re different. Everyone knows you’re the only one You-Know – oh, all right, Voldemort – was frightened of.”
_{^{- Harry Potter and the Philosopher's Stone, Chapter 1 (The Boy who Lived)}}

Dumbledore also tells Harry to use the Dark Lord’s name because fear of a name increases fear of the thing itself.

"Sir?" said Harry. "I’ve been thinking … Sir – even if the Stone’s gone, Vol– … I mean, You-Know-Who –"

"Call him Voldemort, Harry. Always use the proper name for things. Fear of a name increases fear of the thing itself."
_{^{- Harry Potter and the Philosopher's Stone, Chapter 17 (The Man with Two Faces)}}

Dumbledore founded and led the Order of the Phoenix.

"Is anyone going to bother telling me what the Order of the Phoenix –?"

"It’s a secret society," said Hermione quickly. "Dumbledore’s in charge, he founded it. It’s the people who fought against You-Know-Who last time."
_{^{- Harry Potter and the Order of the Phoenix, Chapter 4 (Number Twelve, Grimmauld Place)}}

It’s likely he encouraged the Order to use the Dark Lord’s name as well, because Greyback mentioned that the Death Eaters were able to track the Order by tracing the use of the name “Voldemort”.

"You know who used to like using the Dark Lord’s name, Weasley?" growled Greyback. "The Order of the Phoenix. Mean anything to you?"

"Doh."

"Well, they don’t show the Dark Lord proper respect, so the name’s been Tabooed. A few Order members have been tracked that way."
_{^{- Harry Potter and the Deathly Hallows, Chapter 23 (Malfoy Manor)}}

Lupin was in the Order - he was allowed in meetings for Order members only.

Harry made to follow Lupin, but Mrs. Weasley held him back.

"No, Harry, the meeting’s only for members of the Order."
_{^{- Harry Potter and the Order of the Phoenix, Chapter 4 (Number Twelve, Grimmauld Place)}}

Lupin was also in the original Order, since he’s in the picture of the original Order members that Moody brings to show the others.

“Better dead than what happened to them … and that’s Emmeline Vance, you’ve met her, and that there’s Lupin, obviously … Benjy Fenwick, he copped it too, we only ever found bits of him … shift aside there,’ he added, poking the picture, and the little photographic people edged sideways, so that those who were partially obscured could move to the front.”
_{^{- Harry Potter and the Order of the Phoenix, Chapter 9 (The Woes of Mrs. Weasley)}}

As he was in both versions of the Order, Lupin likely learned over the years to use the Dark Lord’s name. Sirius, another member of both the original and reformed Order, also did.

“I mean, all they’ve really said is that the Order’s trying to stop people joining Vol—’

There was a sharp intake of breath from Ron.

‘—demort,’ said Harry firmly. ‘When are you going to start using his name? Sirius and Lupin do.”
_{^{- Harry Potter and the Order of the Phoenix, Chapter 6 (The Noble and Most Ancient House of Black)}}

It seems to take people a while to get used to saying the Dark Lord’s name, so it’s more likely that members of the original Order, who’d been hearing Dumbledore tell them to say it for several years would be willing to use it.

Answer 3

He has been the member of the Order of Phoenix since the first war with Voldemort. And as we know from Harry Potter and the Deathly Hallows - members of OoP are not afraid of using Voldie's name, which is the reason some of them even get caught, as Greyback mentioned.

"Left, 'ave you, ginger?" said Scabior. "And you decided to go camping? And you thought, just for a laugh, you'd use the Dark Lords name?"

"Nod a laugh," said Ron. "Aggiden."

"Accident?" There was more jeering laughter.

"You know who used to like using the Dark Lord's name, Weasley?" growled Greyback, "The Order of the Phoenix. Mean anything to you?"

"Doh."

"Well, they don't show the Dark Lord proper respect, so the name's been Tabooed. A few Order members have been tracked that way. We'll see. Bind them up with the other two prisoners!" _{Harry Potter and the Deathly Hallows, chapter 23, Malfoy Manor}

Moreover, Lupin was one of the greatest Dumbledore's followers, and we all know what Dumbedore says about using "You-Know-Who" instead of his name.

"Call him Voldemort, Harry. Always use the proper name for things. Fear of a name increases fear of the thing itself." _{Harry Potter and the Sorcerer's Stone, chapter 17, The Man with Two Faces}

Answer 4

I think there's a very subtle difference between the characters who used "Voldemort"early on and those who don't. Eventually most (but not all) members of the OotP use it, but it certainly wasn't a requirement (Tonks, for example, doesn't use Voldemort).

For the early books, the characters who use "Voldemort" are Harry, Dumbledore, Sirius, and Lupin. These are some of the most exceptional wizards in the universe and 3 of the 4 fought in the First War. Presumably, they all have extensive experience with fighting the Dark Arts. They are also brilliant and I think that intelligence underpins the choice to use the word because they are never afraid of the name itself. Dumbledore knew Voldemort and never feared him. Sirius and Lupin were young guns during the First War, they cut their teeth fighting Voldemort. The people that Harry admires the most all say Voldemort, so he does, too. This isn't to say that Tonks, Moody, and Prof. McGonagall aren't great, smart, or brave, but I do think there is a very subtle line drawn between characters who say Voldemort at first and those who do not.