"It's tiring though" Jihwan stretches his body on the seat.
"Well, you're a professional surgeon anyway," Soohyuk chuckled.
"I can't regret now isn't it?" he asked, Soohyuk just thumbs up and nodded.
Jihwan saw Soohyuk tidying up his place.
"You're going out?" he asked.
"Yeah, I wanted to take a look at my dog, she must be very hungry at home," he said.
"What a nice owner you are~" Jihwan laughed and Soohyuk just rolled his eyes and then walked away.
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25, A professional and popular surgeon, Yoon Jihwan. He has been interested in biology since 13 cause he likes to know what's in the human body, kinda like a creep man but he managed to be a successful person now.
Jihwan lived alone but Soohyuk , a psychologist and also a specialist doctor, was his best friend since highschool. They met when Soohyuk saw Jihwan sitting alone at the back of the lab. They started to talk and share secrets a lot.
Since then, they used to be together, even got to the same workplace.
"I hope he'll be a bit gentle with her," Jihwan mumbled while driving back home.
…
Once he reached home, Jihwan laid on the bed with his hands resting on his forehead. 
"I should take a quick shower at least, today I'll get some rest from doing it I guess..." he started to get into shower and suddenly felt like someone was looking at him.
He turned around and looked around his room, there's nothing and he tried to snap the curtains but there's nothing behind it.
"Was that just my instinct?" he mumbled, then continued to go showering.
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On the side of dimensions, a few hours ago…
"Oppa, what is human? Why do I need to go to earth?" Dahee asked.
"Since you refused to get married to the demon king, I want you to do some research about humans," he said, The Devil of Gluttony Kim Daehyun.
"But why? I mean why do I need to do research about humans?" she didn't understand.
"Dahee, if you don't want to be the demon king's wife, you'll be a succubus...you need to learn what is human and what is earth" Daehyun said, snapping his paperwork.
Dahee sighed "fine!!" she stomped out but at the same time, she used her magic to steal one of the books from Daehyun's reading room.
"What an easy~" she said and got into her room, snapping through all the pages and there's a piece of paper that fell down.
"What is—" Dahee was frowned to see a picture of her childhood that looked like sitting in heaven with her family.
Suddenly, Daehyun appeared in front of her and grabbed the book along with the picture.
"Don't steal anything from my room!!" he scolded her but she ignored it and wanted to know about the picture.
"What's that just now?!" she asked, pointing at the paper that Daehyun hid behind him.
"N-nothing…" he looked away.
"Don't lie to me! I saw that picture clearly with my two eyes!! That's our family!! In heaven!!" she yelled.
Daehyun was stunned and looked at his sister. He knew that he couldn't hide anymore but the promise he made did not allow him to say anything. It's not the time yet to tell her the truth.
"Dahee, look me in the eyes, I'll tell you everything…" he said and their eyes met.
But Daehyun secretly snaps his finger and erases Dahee's memory of the picture. Then, she blinked and looked at him confused.
"W-what, why are you here?" She looked at Daehyun weirdly.
"You stole my book" he said and walked away.
He closed the door and Dahee sat at the bed "he did that again, trying to erase my memory every time I found something…" Dahee sighed.
For 5000 years in hell, everytime she found something that she didn't know and wanted to ask her brother, Daehyun would always snap his finger and erase her memory.
But after 5000 years, Dahee started to learn and hide the secret that she found alone because there's one time when she got her memory and the truth of what her brother did to her.
"What's the secret that you're trying to hide, oppa?" Dahee looked at his brother are looking at the picture that she wanted to know what's the story behind it.
"If you don't want to tell the truth, I'll find it by myself" she said and sneaked out from the castle, walked along through The Forest of Thirst where all the creepy creatures that were trying to suck demons blood.
Dahee tries to walk through as fast as she can. She can't fly on top of the tree or Daehyun will know about it and grounded her.
After all the struggles and obstacles, she is finally at the main door of hell, where it leads all the angels and demons to the other dimension. Earth was one of them.
But one more obstacle was in front where there's two big and tough guards who controlled the door.
"Luckily I'm a good liar~" she mumbled and confidently walked towards them.
"What's your deal here?" one of the demons asked in a very deep and aggressive voice.
"I need to go to the earth, studying about humans," she said without stuttering.
"Who gave you—"
"My oppa, The Demon of Gluttony Kim Daehyun…" she said in poker face.
"Well, if that's what The Demon of Gluttony ordered, we'll give you the pass and hope you'll fail" the other demons said.
"That's rude though" Dahee walked out and was stunned with the door of Heaven in front of her that was milky, silky white but unfortunately, she can't even touch it.
Talking about other business, Dahee flies to the earth and transforms into human form. She walked from the dark tunnel and swarmed in the crowd.
She then gets into an unknown house from a window.
"Is this the human's house that, oppa said?" she asked then looked at the picture on the desk, beside the bed.
"A young and quite handsome boy, he had a good look when kid…" Dahee traveled around to his house and suddenly saw a room that only had the door locked.
"What room—" she twisted the doorknob but it seemed to be locked. The harder she twists, the harder for the doorknob to get open.
"What kind of door is this?! Even if I use my power, it'll not open…" she said, then just left the room with her curiosity surrounding.
"So, where should I hide?" she said then looked everywhere again.
"This is so troublesome, why can't demons use invisible power when they are on the earth?! It'll make my job easier" she said and saw the bed that left a lot of space under it.
"It seems like I'm gonna be a children's folklore…" she sighed then got under the bed.
"Well, it's not that bad…I can be here for about 2 days…" she said then someone came in.
"Was he the owner?" she mumbled then looked at his foot. 
She looked at him and opened his coat then shirts but she couldn't look at his face.
"Look here…" she mumbled because of curiosity.
But then, that guy turned around and went to the curtain but Dahee was confused with what he's trying to do.
"Why did he snap the curtain so aggressively?" her heart whispered.
…
Jihwan walked out with a towel on his torso and his hair. He sat down at the bed and looked at the phone. It's more early than usual and he needs to do it now but he is too tired.
"I'm so lazy to go out again…" he said then someone called.
"Yow, Jihwan!" it's Soohyuk calling.
"Yes Hyuk, what do you want?" he asked.
"Don't be so cold! Wanna drink? At my house" he asked.
"I've told you that I can't drink soju…no alcohol anymore" Jihwan slid his t-shirt.
"Who said alcohol? I know you're tired and have no one, so because of that I'm inviting you" Soohyuk pouring a drink into the glass.
"I'll be there but only for a drink, not to help you with your work" Jihwan gets his car key along with coat.
"What a miss, I want you to teach me how to make those things better and more clean," he said.
"I can but I don't want to touch it, I'll get dirty and need to shower again" Jihwan complained.
"What a hygienic person you are, are you sure that you don't have OCD?" Soohyuk asked.
"I'm a surgeon, hygiene was part of my routine and it's kinda influenced my daily life, that's why…" 
"Ouh, I forgot that you're a surgeon! You're right and don't worry, I don't need you to touch it, just give me the order" he smirked then they ended the call.
Jihwan drives his car but still feels disturb by something at his house
"I'm sure there's someone…" he mumbled.
Meanwhile, Dahee was crawling out from under the bed.
"I should be more careful…" she said then laid on the bed.
"I shouldn't have laid here but…I’m…sleepy" she said and turned to demon form.
"I can't let the feather fall…" she said, then accidentally took a nap.
…
Meanwhile…
"You're here Hwan!" Soohyuk waved from afar.
Soohyuk served him a drink.
"You can have it as much as you want but teach me that thing!" Soohyuk said in excitement.
"You know what? You look like a dog wagging his tail" Jihwan said calmly.
"Well, I have a lot of dogs wagging their tails~" he said, smirking.
"What do you want me to teach you?" he asked.
"You know that the heart can beat for up to 4 hours in the ice box right?" Soohyuk asked.
"Yeah"
"Then what if we didn't put it in the ice box, if the warm was the one that keep it beating, how long it can beat without an ice box"
Jihwan was thinking, then had a few sips before explaining. 
"Actually, if you put the heart without an ice box, the cells will run out of energy and will last for 3 to 5 minutes only, that's why we put it in an ice box," Jihwan said.
"And it's the cold that keeps it beating, not the warm, the hearts need to warm up before it beats at the normal rate in the body…" Jihwan continued.
"Then, why put it in an ice box? I mean, the inside of a human's body is warm and the hearts can keep beating?
"It's not about the temperature, it's about the function. Your heart is pumping the blood all over your body as the blood contains oxygen. As long as you need the oxygen, your heart will work for it" he said and Soohyuk just nodded.
"But if your brain dies, your organs will stop and your heart also doesn't have any function in your dead body, so that's why it stops too. Because of that, the surgeons will do the heart transplantation when the body is still alive and fresh" he said.
"Then, how can a person still survive after their heart is removed?" Soohyuk asked.
"We used a ventilator and put the heart as fast as we could, that's how the surgeons do," he ended.
"Then…can you demonstrate it to me?" Soohyuk asked.
"Haven't I told you that I'll not make myself dirty?" he asked.
"But, it'll not make you dirty~ I promise!!" he said and begged him.
Jihwan sighed and they got into a room. Soohyuk was excited and they got changed. The things that they're doing was a secret that they always do.