"...Professor Quirrell!"
Harry's fear-filled exclamation made William and Lupin instinctively glance at each other, seeing a hint of confusion in each other's eyes.
Professor Quirrell...William remembered hearing this name, and most of the time, it would appear alongside last year's Defense Against the Dark Arts professor, Gilderoy Lockhart. Especially in the past two days, because Lupin's behavior was too "normal," all the past Defense Against the Dark Arts professors were being brought out for a roasting—
But, if this person was Quirrell, then what was with the face on the back of his head?
...And where was his nose?
"Harry! Repeat after me!"
Watching the "smiling" Professor Quirrell approach Harry step by step, Lupin harshly called to wake the boy up, gesturing with an empty hand as though waving a wand, "Riddikulus—"
"Riddikulus!"
Reminded, Harry instinctively grabbed his wand, and as his hoarse voice shouted the charm, there was a sharp crack like a whip snapping. "Quirrell" began to stagger; he first uncontrollably took several steps back, then a piercing scream almost lifted the roof of the Room of Requirement—
The scream came from the mouth of the person facing away from the three, as "Quirrell's" figure began to hunch. Blisters resembling dragon pox appeared on both faces, as if a corrosive gaze was starting to emanate, and the man's body began to melt inexplicably.
"Boom—"
Then, accompanied by a soft sound, "Quirrell" turned entirely into a mass of grayish-white mist, which completely dissipated into the air.
"Bang—"
The drawer on the table closed with a snap.
"...Wow, disappearing like that isn't interesting at all."
In the silence that followed, Lupin's smile seemed a bit dry, his thoughts somewhat chaotic—he had originally thought that Harry's greatest fear might be Dementors, which was why he had suggested this, but the result was clearly different from what he had expected. But...why was it a Defense Against the Dark Arts professor from two years ago?
Could it be... A thought occurred to the man, a guess he had previously dismissed during a conversation.
"Interesting?"
Harry looked up, puzzled. He was still somewhat shaken, even though two years had passed. The scene before the Mirror of Erised was something he might never forget; that was the first time he faced...Voldemort directly.
And to Harry, this memory certainly could never be described as "interesting."
Seeing that Harry's understanding had obviously deviated from what he meant, Lupin shook his head and, pointing to the nearby drawer containing the Boggart, explained—
"A Boggart is a shape-shifting magical creature that will determine the thing a person standing in front of it fears most, and then it will take that form... The real way to defeat a Boggart is actually with genuine laughter. We typically need to use imagination to force it into becoming a comical image; I was just about to tell you that..."
Lupin's tone was uncertain, and then he looked toward William for assistance—Boggarts, while frightening, aren't actually lethal. A spider conjured by a Boggart won't secrete deadly venom, and the screaming banshee it turns into won't actually kill anyone. So Lupin had only been demonstrating the spell to Harry—
Real teaching hadn't even started yet, but the Boggart was already defeated with a wave of the wand—
This was a very odd situation, as Lupin had never seen anyone handle a Boggart this way, and no Boggart had ever been defeated by anything other than joyful emotions. But clearly, what had just happened with the "Professor Quirrell" had nothing to do with the term "comical," so Lupin, unable to figure out why, turned to William.
"...It almost burst from overeating."
"?"
"The characteristics of Boggarts and Dementors are essentially similar, but with some differences. Although Dementors are also very afraid of the spells wizards use to release joyful emotions, what they consume is precisely the happiness in the hearts of wizards or Muggles. But Boggarts are different; they eat negative emotions, with fear being the chief among them."
Lupin, of course, knew these basic principles, so William was mostly explaining for Harry's benefit.
Similarly, after William's explanation, the two broken threads in Lupin's mind successfully connected. At this point, he clearly understood what William meant by "burst from overeating."
It meant that the negative emotions Harry just produced directly overfed the Boggart, and with the addition of a spell, it was forced to make a quick exit; otherwise, it might have burst from sudden overeating.
So...could this be a major problem?
Lupin wasn't stupid, and even a fool could see something was wrong. Harry's behavior was definitely abnormal. How could an ordinary, unremarkable little wizard who had only defeated the Dark Lord at one year old possibly burst with such strong negative emotions?
Blowing up a Boggart?
Lupin's eyelids couldn't help but twitch as he glanced at William, his expression unmistakably clear—Is this right?
This is definitely not right.
William obviously realized this too. Such a huge amount of negative emotions could not possibly have come from Harry himself, meaning the source could only be that extra entity residing in Harry's mind... But, why would it react so strongly to Voldemort, who was on the back of Quirrell's head?
Could it be that it really is Little Tom?
After seeing himself become that figure in the future, the resentment that arose doesn't seem entirely unreasonable. After all, to give credit where it's due, before Tom thoroughly started systematically researching Dark Arts and was assimilated, he was somewhat handsome—
William had heard quite a lot of gossip about Voldemort from Dumbledore, like the story of him being kept by "Rich Lady" Mrs. Smith...
From this, William couldn't help but feel somewhat fortunate that he hadn't, in pursuit of power, dabbled in Dark Arts. Before mastering Emotion Magic, using those powers with restraint was essential; messing around could be as trivial as affecting one's health or as serious as ruining one's appearance!
Someone accepting themselves without a nose...too terrifying.
Of course, at the same time, William also realized that when doing many things, one must be cautious of old Dumbledore, who was really quite the old voyeuristic magic master.