No matter what, Zelo was going to die. That was something planned from the beginning and one of the scenes I looked forward to the most to write. What happened afterwards though kept changing.
My first thought was for them to simply download Zelo in to the ship's computer. Then they could repair his body and then put him back in to his body. Which of course would bring about a funny moment of Zelo talking to Jisun through the ship. Also several hours hooked up to the ship and running diagnostic tests to be sure that he didn't change again. I decided that wouldn't work for two reasons though.
Firstly because Zelo is a cyborg, not a robot. Robots are 100% computer. Zelo has a brain and other organs. It's more like his computer parts were added to his body. So there technically wouldn't be a way to download him because his brain would be where his memory and personality are. Secondly; biological parts decay. It would have been almost 36 hours between his time of death and when they got him to the ship. Internal organs start decaying within 24 hours, thus making his body beyond repair. Of course I could have some sort of 'miracle' Mato technology that could regrow parts of him that already started to decay but I figured that was a little too much of a stretch for science. Dead means dead.
Since my first idea wouldn't work, I came up with idea number two. Giving Zelo a new body and then downloading a pre-saved version of him in to it. I thought that Zelo could interface with the ship and leave a sort of 'diary' behind every time. A record of everything that has happened to him. He wouldn't exactly 'remember' everything in detail but enough for it to feel like he had experienced it. Of course that meant he wouldn't remember Earth though since BAP were attacked and the bad guys took the ship. His last diary entry would have been right before he landed. So there would be a scene of them telling him everything that happened and him coming to terms with what he did to Jisun.
Ultimately, that idea was scrapped as well. I'll be perfectly honest as to why, though it seems really harsh and bad on my part. I got rid of him cause of some story comments. My interpretation of some comments made me feel like his death wasn't really mourned. That he either wasn't dead or that he could easily be brought back so killing him wasn't a big deal. I was so upset by that and at one point, even almost started crying out of frustration. I could be wrong of course and the people who left those comments could have mean something completely different but those are just my feelings. I became jaded and just wanted him out of the story permanently so that's what I did.
Leaving the group as five members however seemed wrong. I said in the fic before that every unit had a ZELO for certain tasks which meant that I would have to create a sixth member to replace Zelo. That seemed even more wrong though. So I came up with a 'twin' who would look like Zelo but be a different person. That way it filled the missing gap but Zelo still didn't make a comeback.

Here is the way I would have brought Zelo back with him getting a new body and interfacing with the ship. The first part happens on the ship the next day, after Jisun and Yongguk spent the night together. You'll have to imagine the conversation about Zelo between Jisun, Himchan, and Jongup never happened. And since Yongguk has an important thing to say regarding Zelo, you also get to see Yongguk and Jisun say goodbye.

Jisun stirred awake but kept her eyes shut. She reached out a little with her right hand but felt only blanket. Then she opened her eyes and saw that Yongguk was gone. She inhaled sharply and quickly sat up before glancing around the ship. When her gaze landed on Yongguk, relief flooded through her.
"Hey." Yongguk said as he got up from where he was sitting. He hurried over to her and knelt down beside her. "It's okay. You're fine." he assured her, noting her panic.
She swallowed and wrapped her arm around him in a hug. "I was worried about you." she quietly said as she felt her heart still racing.
"I had another dose of medicine just a little bit ago." he informed her. "So I'm fine. I promise."
She forced herself to take a slow, deep breath and nodded. Then she let him go and he pulled back a little. She glanced around and noticed that the only other person in the ship was Himchan. "What's going on?" she asked.
"Youngjae and Jongup are finishing some stuff up." Himchan replied. "Making sure everything is taken care of."
She bit her lip and glanced between the two males. "And... The..."
"You don't have to worry about anything." Yongguk insisted. "I would never leave if I didn't think you'll be safe."
She stared at him and saw the sincerity in his eyes. She gave a nod of her head, silently telling him that she trusted him. Then he moved so he was sitting next to her.
"As soon as the others get back, we're... We have to go." Himchan stated as he stood up.
Jisun felt her heart skip a beat and she clutched at Yongguk, making him move closer to her. She knew it needed to happen though, even with how much she hated the idea of being away from him. He needed help. They all did.
"I'll give you guys a moment." said Himchan quietly. Then he turned and went to the far end of the spaceship. That's when she noticed that the door was open and Himchan stepped outside.
She stared at the trees she could see through the doorway as well as the sunlight that shone in. Then she turned her attention back to Yongguk. "Daehyun?" she hesitantly asked.
"He... is still on life support." Yongguk answered solemnly.
She pressed her lips together. Jongup's words echoed in her mind, "Don't lose your heart". And the ache in her heart signaled to her that she hoped he would live. Which of course made her think about the member they had already lost. She let her gaze drift over to the pod that held Zelo's body before looking at the floor. "What... What happens to..."
There was a moment of silence before Yongguk said, "We're going to try to bring him back."
She gasped and quickly looked up at him with wide eyes. Bring him back?
"Wait, let me explain." he hurriedly told her. "It's not a guarantee."
She opened but then closed it and waited for him to continue.
"Every ZELO unit interfaces with the ship and leaves a record." he explained. "Not only everything that had happened, but their thoughts as well. Sort of a diary almost. Youngjae checked Zelo's record and saw that it was very detailed. Every little thought or idea he ever had, he recorded it."
She furrowed her eyebrows as she tried to understand what this meant.
"On Mato," Yongguk continued. "We can have a new body made. Then we're going to try and download his record in to the computer chips of that new body. It will tell his brain that an event happened and his brain will believe it. He won't have any visual memories of course but in a way, he'll remember it."
"But... It's not a guarantee?" she questioned.
He took in a deep breath and sighed. "No. The brain and the computer chips have to work in harmony. If there is too much of a gap in the record, he could not understand parts of it and it'll cause him to malfunction. And there's always the chance that even if it worked, he still won't be the same person. He could have a different personality depending on the synergy between the two parts."
She slowly nodded her head as this new information sunk in. It was hard to wrap her mind around, trying to understand how matoki technology worked. But this was a chance. No matter how small that chance was, she'll still take it.
"So... he wouldn't remember me." she uttered.
"No." he said. "The last time he interfaced with the ship was before we landed." He reached over and gently touched her left hand. The hand that Zelo had stabbed with the knife. "He won't remember any of this."
She bit her lip as the vivid image of being in that room came to her mind. Then the look Zelo had given her when he said he didn't want to hurt anyone again.
"Can we... not... tell him." she quietly begged.
He gave her a concerned look as his fingers brushed the bandage around her injured hand. "No."
dropped open in surprise. "But.."
"I know how it feels." he stated, interrupting her. "To have your memories taken away." He let her hand go and rested his hand on her leg. "No matter how painful memories are, they are still a part of you. We can't take that away from him."
A noise made them look over towards the door. Himchan was there as well as Youngjae and Jongup.
"It's time." Himchan said as he glanced between them.
Jisun felt her eyes watering as she turned back to Yongguk. He was looking at her and he brought his hand up to cup her face. "I'll walk you home."

As it turned out, the ship wasn't too far from her house. The walk back to her home with Yongguk, hand in hand, was much shorter than she wanted it to be. So when they stepped on to her back porch, she found it hard to let his hand go.
"Promise me again." she pleaded as her voice cracked.
He squeezed her hand and wrapped his free arm around her. "I will come back to you." he whispered in her ear.
Tears leaked from her eyes to roll down her cheeks. With a shuddering breath, she finally let go fo his hand. "You better go." she whimpered.
Instead of pulling back, he simply put his other arm around her and held her tightly. She squeezed her eyes shut and buried her face in to his shirt.
"I swear I'll come back for you." he promised once more, his own voice straining. Then he pulled back and reached up to his neck.
As she watched up, she could see how red his eyes were. That he was doing everything he could to not cry. He lifted the chain off of his neck that held his dog tags. He unclasped it and slipped one of the tags off. Then he clasped the chain and put it back on.
"Take this." he told her and grabbed her right hand. He pushed the tag in to her palm before closing her fingers around it. "Something to remember me by."
"I'll never forget you." she uttered.
He met her gaze and she saw a couple of tears finally fall from his eyes. Then he swiftly moved forward and kissed her. She closed her eyes and eagerly kissed him back with as much passion as she could muster. Pouring all her love in to their last kiss as he did the same.
When the broke apart, he hugged her again. "I love you."
"I love you Yongguk." she told him. Then she swallowed and took a shaky breath. With what little strength she had, she pushed him away. Somehow, she knew it was the only way. "Go." she insisted with a whine. "Just hurry back."
He pressed his lips together and nodded. Then he stepped backwards, moving away from her. Only when he reached the edge of the porch did he finally tear his gaze off of her and turned around. He hurried off the porch before running across the yard. Without looking back.
"Cause if he looked back, he wouldn't leave." she thought to herself.
She waited until he disappeared in to the forest. When she couldn't see him anymore. Only then did she turn away as well and open the back door. She stepped inside and closed the door behind her. Then she leaned against it as she slid down to sit on the floor.
She stared at the spot on the mud room floor where Yongguk had laid when she first brought him here. Dragging him from the forest while a storm drenched them. Then her vision clouded and she let out a painful sob. Her hand tightened around the dog tag in her hand as she let herself cry.

(Fast forward to Jisun in the ship. After her and Yongguk are reunited.)

"Hyung, why is there a human on the ship?"
Jisun jumped and spun around to see that the display of affection was being watched. She immediately blushed and glanced at Yongguk to see him blushing as well. Then she turned her attention to the person the question had came from. It was Zelo. Exactly as he had looked before. Even with red hair. Which had to mean it worked. His new body accepted the record that was downloaded in to the computer part in his brain.
"This is Jisun." Himchan explained to the youngest while grinning happily at her and Yongguk.
"Ohhhhhh." Zelo replied and slowly nodded his head.
Jisun bit her lip as she turned and picked up the strawberry jam. Then she carried it to the boy and offered it to him. "This is for you." she said as he hesitated for a moment before taking it. "It's nice to meet you. Again."
He glanced down at the jar for a moment before looking at her. He pressed his lips together as held the jar with one hand and slowly reached out with the other hand. He lightly grabbed her left hand and when she didn't pull away, he brought it up to examine it.
She knew what he was looking at. It was something that she had gotten use to seeing the past year. A scar in the shape of a line in her palm.
"I'm sorry." he quietly said as he continued looking at it, his eyes filling with sadness.
"It wasn't you." she insisted.
He finally looked up and met her gaze. "It was though. I mean, in a way no. But at the same time, I..."
She moved her hand so she could grab his hand. "It wasn't you." she reiterated as she squeezed his hand. "They turned you in to someone you're not. I know where your heart is. And I know it wasn't you."
He blinked at her for a second before lightly blushing. A sheepish smile formed on his face as he finally nodded in agreement.
She let go of his hand and pointed at the jar. "That's strawberry jam I made. You really liked it before."
The smile on his face grew bigger. "If you made it, then I'm sure I'll like it."
She smiled as well and chuckled a little. Then she jumped a little as Zelo suddenly cried out.
"Oh! Can I call you noona?" he asked with a hopeful glint in his eye.
She stared at him in surprise before bursting out with laughter. "Of course." she replied as her heart gave a little flutter.
Sounds of laughter mad her glance around at the other on the ship. Everyone was smiling at her and she smiled back at them. She realized just how much she missed them as well and not just Yongguk. But there was someone missing.
Her smile quickly disappeared as she turned and looked around the ship. "Where's Daehyun?" she questioned as she turned back to face them.

(And then continuing on from there. With Zelo of course hogging the jam, skateboarding, and enjoying strawberry ice cream.)