Your scene has a strong sense of mystery and personality, and the banter between Ark and Sunshrite is entertaining. I edited it for grammar, clarity, pacing, and more natural dialogue while preserving your style.
Writing
I closed my eyes, and within a couple of minutes, I drifted into sleep.
Since this wasn't an ordinary dream, those few minutes felt more like a transition from my bedroom into that strange realm.
When I opened my eyes, I found myself standing once again within the endless white space.
A single wooden door stood before me. The familiar slime was there as well, but to my surprise, another figure had joined us.
These entities were the only things that brought color to this endless void: the evergreen slime, the old brown wooden door, and a small yellow mushroom-like creature that resembled the sun.
"[Greetings, Master. How have you been during your time away?]" the slime asked.
Despite his robotic voice, his appearance was nothing more than that of an adorable little slime. He didn't hop around like ordinary slimes either. Instead, he remained perfectly still beside the door, like a responsive NPC waiting for the player's next inquiry.
"Oh, hello, Ark. I've been doing well these past few days. And who might this visitor be?"
"[I would be delighted to introduce him. He is Sunshrite, the Sun God's companion.]"
"I'm not a pet!" the little mushroom protested.
"I have a voice, so shut up, you slimy turd! I am Sunshrite, the Great Sun's messenger. It's a pleasure to meet you, Sir Will!"
"[What did you just call me?!]"
The two immediately began hurling insults at each other.
Despite their adorable appearances, they were surprisingly fierce and completely unrestrained. I let out a weary sigh. I had no idea where either of them had come from. I had asked Ark before, but all he ever gave me were long-winded, pretentious explanations that made little sense.
"Alright, that's enough, you two."
The moment I tried to stop them, they started fighting instead.
Man... they're becoming a real headache.
Maybe I'll ask another time.
My attention shifted toward the lone wooden door. I wondered what kind of world lay beyond it. As I reached for the doorknob, Ark and Sunshrite suddenly spoke.
"[Have a safe journey, Master.]"
"Best of luck, Sir Will!"
I paused. It seemed they had finally stopped bickering.
Turning toward them, I asked, "Do either of you know what lies beyond this door?"
"[As I have told you before, Master, I do not know. I only know that what lies beyond are the collective memories of your soul's previous possessor.]"
"My answer is the same, Sir Will."
Right... just like the book had said.
It seemed that the memories of a reincarnated soul's previous life were gathered within this place. What those memories truly contained remained unknown. Ark had also told me that, back when this realm was still shrouded in darkness, I had already relived they're present life's memories through those dreams.
"I've been wondering..." I said, looking at the little yellow mushroom. "Where exactly did you come from, Sunshrite?"
"At last, Sir Will, you have acknowledged my presence. As I have already introduced myself, I stand before you as the envoy of my master, Sol, the Great Sun God of Creation and Light. I have also come because of the immense holy power that slumbers deep within your soul."
The tiny yellow mushroom proclaimed his introduction proudly, his puffy eyes gleaming with confidence.
"My immense holy power?"
Ever since childhood, I had been a sickly person. I would cough up blood without warning. If Mother hadn't helped me suppress this power while teaching me holy magic and various techniques, I might never have survived.
Lately, however, the symptoms had begun to return. Now I suffered random headaches instead.
Yesterday, I finally discovered the reason.
After regaining the system, notifications had begun appearing one after another.
[A devout worshipper has offered a sacrifice. Holy Power increased.]
[You have received new worship due to your heroic deeds. Holy Power increased.]
There were well over a dozen messages, yet every one of them said essentially the same thing.
My holy power was increasing.
I had absolutely no idea what was happening. Ark kept insisting that I had inherited the powers once wielded by my soul's previous possessor.
The whole situation left me utterly perplexed. Too many strange things were happening at once, and this world was already chaotic enough without adding another mystery.
"I suppose I shouldn't be surprised anymore."
"Correct!" Sunshrite exclaimed. "If you wish to know where your holy power is stored, simply look around you."
I did exactly as he instructed.
Yet all I saw was the vast, endless expanse of white. No walls. No ceiling. No floor. No edges. No corners.
Completely puzzled, I frowned.
"But... there's nothing here."
"Do you not see it? It's everywhere!"
Just as I was about to refute his claim, Ark suddenly stretched part of his body into the shape of a hand and smacked Sunshrite on the head.
The hit wasn't particularly strong. The mushroom simply bounced like a balloon.
"[You idiot! Can't you see that Master can't perceive it?!]"
"What did you just call me?!"
Are they really going to start arguing again?
I didn't have much time before daybreak. If this dream dragged on any longer, I'd wake up exhausted just like last week.
"Enough!"
My voice carried a hint of authority.
"Why do the two of you keep fighting?"
They answered in perfect unison.
"Because he's a thief!"
The two glared at one another before immediately resuming their argument.
"[He's mine! He is my master!]"
"No! He is the Sun God's vessel! He doesn't belong to you!"
"...Sigh."
I shook my head in resignation.
"They're hopeless."
I wasn't going to waste any more time.
Grasping the doorknob, I swung the door open.
A brilliant flash of light flooded my vision.
Just like before, an invisible force pulled me inside.
The white world disappeared.
The next thing I knew, I had entered the very heart of the dream.
A vivid dream.
Unlike last time, this time I had possessed the body of a human.
"Where am I?"
I looked around.
It was a modest room containing little more than a bed. Looking into a nearby mirror, I saw the face of a middle-aged man with a scar running across his forehead.
As I examined my surroundings, a familiar mechanical voice echoed inside my head.
[Ding! Dreamscape memory simulation active... Status: Operational.]
"That voice..."
"It must be Ark."
A couple of style suggestions:
Since this is a fantasy novel, avoid modern gaming terms like NPC unless your world intentionally blends fantasy with game mechanics.
Consider choosing one spelling and using it consistently. For example, Sunshrite appears throughout; if it's the final name, keep it consistent everywhere.
Your dialogue is one of the strongest parts. Leaning a little more into each character's unique voice will make Ark and Sunshrite even more memorable.