In the life of Cybertronians - they are given one sparklate to last throughout their entire lives. If one of the partners were to go offline... that bot would be alone with no other chosen sparkmate.
That's what happened with Optimus Prime when he lost Elita One.
He went into a deep depression before the war broke out. After the war, he focused on the survival of his kind before he and his team were sent to Earth to hide from the Decepticons.
After some time, Optimus has a weird feeling in his chest plate. Something wasn't ... right? He knew something was off, but would always put it off and focus on missions and his team.
Until SHE came along.
There was a fight at one of the Energon mines with his team and the Decepticons. Optimus was really off his game during this time, but that was because he spotted a human male and younger human female trying to escape without getting caught in the crossfire.
The human girl was running close behind her father, but once her foot stepped on some loose rock, she lost her balance and began screaming. Optimus quickly blasted one Decepticon head off before jumping upwards and catching her midair.
When he landed on his peds safely, the human girl's <eye color> eyes made contact with his blue optics, and there was a small spark that ignited between the two of them.
This moment was when Optimus noticed she had a robotic left leg and robotic right arm on herself. They stretched all the way up to her hip and shoulders.
After the battle died down and Bulkhead retrieved the father carefully, the Autobots brought the two humans back to their base - the human girl, whom Optimus came to know was Y/N, was sleeping on the couch. He could hear what sounded like air leaving vents every time she exhaled.
The father than began to explain that his child was born very premature and the mom did not make it pass child birth - his wife's dying words were to do everything in his power to bring their child to life and keep it alive no matter the cost. When Y/N was born, she was missing her right arm, left leg, and majority of her human organs. Though against his own morals, he did not want to dishonor his wife's wishes and brought his daughter to his lab, where he constructed her human body parts and organs out of strange metallic material his own team discovered days prior, and that was what brought his daughter to life.
Ratchet did a full body scan on Y/N as she slept and widened his optics. "Optimus..." he pointed, speaking in a soft tone. "This girl is made up of Cybertronian technology."
Suddenly a sharp pain was felt in Optimus' right shoulder. He grunted and grabbed it with his left cervo before glancing over at Y/N, seeing her withering and crying in pain for her dad. "Dad- my arm!"
After some time of adjustments and getting Y/N back to sleep, the father admitted that with his age and the lack of knowledge with the technology used, he would not be able to take care of you much longer. He wishes he couldn't have done what he did, but he did not want to dishonor his wife's dying wish. He even mentioned that you have always forgiven him for doing such a thing, but he could never forgive himself.
After a few hours, the father said he needed to leave to run an errand, but he never returned.
It seemed after Y/N woke up from her long rest, the first thing that happened was she laid her eyes on Optimus' optics and that spark was ignited again. They both glanced away from each other, blushing, before Y/N spoke up, "W-Where's my dad?"
Ratchet couldn't help but glance at the other bots, nervously gulping. "He uh.... He hasn't came back."
"Oh." Was all you could muster out. You steadied yourself as you stood up, walking over to the ledge carefully. Your hands gripped the railing bars before you glanced over at Optimus again, feeling that same surge of energy run through your body.
