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Rimaru is settled into life as a slime. His human life feeling more like a distant memory every day led to certain habits in his life that were lost on more humanoid monsters. His form for one; his subordinates largely seemed to assume staying in slime form was a way to conserve energy, or even a way to convey a relaxed state of their ruler. In reality it took no energy at all to maintain his mimics anymore, not since he was newly incarnated and freshly named; and his form was hardly based on emotions (though human features made body language easier for others to read). Rimaru simply enjoyed his slime form. It was his truest form and the one he had come to feel at home in. Human senses had become a novelty to him, dulled and simple compared to the breadth of his magic sense which made him feel connected to his surroundings in a now familiar way.
So yes, Rimaru’s human appearance was largely for the convenience of others. He had found many people responded better to a human form, something they felt they could understand and read visually. Gazel was a pleasant exception among foreign allies as he had known the slime before gaining his human form. In fact in private he was preferred Rimaru in his slime form, claiming to favor the honesty of talking to the slime as he was.
Of course the citizens of Tempest, especially those close to him, adored his slime form. Its likeness was everywhere from architecture to confectioneries. Shion and Shuna took great pride in having the privilege of carrying their master, an indulgence he allowed fondly. These facts should then logically compound to the conclusion that Souei would not mind his slime body when they were together. Unfortunately logic was known to evade even the great and powerful Demon Lord Rimaru from time to time.
When his relationship with Souei began to grow beyond mutual respect and friendship into more romantic feelings, Rimaru began to make an effort to appear human in front of his lover. The oni was largely human-passing so Rimaru assumed he would prefer a more familiar form to interact with and when they introduced intimacy to their relationship Rimaru would change his sexless body to something more compatible with his lover, even so far as allowing Souei to choose if he wanted masculine or feminine traits on his partner.
Rimaru felt very little sexual attraction at this point in his life as a slime. Slimes produced asexually so the biological need for such a drive was simply absent. Of course Rimaru was a fully sentient being with feelings beyond what was strictly necessary for his biology and did enjoy sex with his beloved oni, however he didn’t particularly need or crave that element in a relationship as he did in his previous life.
All in all he was perfectly content with his and Souei’s relationship. He didn’t feel as though there was any lacking element between them until Souei pointed it out.
“Why do you do that?” Souei asked one day as he entered the home he now shared with his love.
Rimaru looked at him, confused, “Why do I do what?”
“Change,” his lord simply stared so he continued, “You change forms whenever I enter the room, no matter what you’re doing or who you’re with you always without fail change into your human form to greet me,”
Rimaru stared into space, clearly deep in thought, as Souei sat next to him on the tatami mats. As he made himself comfortable his lover spoke up.
“I never really thought about it before but I do, don’t I?” The slime continued to look into the distance as he spoke, “I guess at some point I just decided this was the form I used for you… It has it’s perks thought right? I can’t exactly kiss you or hold your hand as a slime,” He waved his hand dismissively to avoid the realization that Souei may have highlighted a deeper topic than he expected.
The oni gave a thoughtful hum as he decided how to respond, “You are aware I do not find your slime body displeasing right? I have failed as both a lover and friend if you have come to believe I do not appreciate all of you,” His tone had taken a more serious tone than the one he usually spoke to Rimaru in when they were alone. He reached for his partner and gently ran his fingers through brilliant blue hair.
His love sighed at the touch, leaning into the feeling. He loved how Souei handled him. Like he was something to be treasured and protected. It was an odd feeling. He had an army of monsters and demihumans ready to fight to their final breath at his command, as well as the power to level armies with solely his own power. Yet with Souei it felt different, it wasn’t obligation or loyalty that caused his fierce possessiveness. It may have started with those feelings but they had since grown into a tender devotion motivated by love for Rimaru as an individual rather than a symbol or idol to be worshiped and served.
“You’ve done nothing wrong Souei, I swear. I’m simply… used to others finding this form easier to communicate with, slimes aren’t exactly the easiest monster to connect with,” He can’t recall admitting that before, he had previously remarked that his human form was for the comfort of the humans around him but to say so plainly that his slime self was dismissed by others was strange.
Souei pulled him in to a tight embrace and whispered into his ear, “Never think my love for you is limited to how you present yourself, if you believe that I would not know you in your true body as well as I do in this one you are not nearly as observant as I was lead to believe,” Sealing his words with a kiss to Rimaru’ temple he loosened his grip to look him in the eyes.
Rimaru looked at Souei with pure adoration and love, as tears welled in his eyes he huffed a laugh and melted out of his human mimic. Cuddling into Souei’s abdomen he whispered, “I’m sorry I was so stupid about this, Souei, you’ve never done anything but see me as I am. It was foolish to think that would change,”
His oni softly kneaded his fingers into his love’s body, “You are many things my love, stupid will never be one of them. Even you are allowed moments of weakness now and again, let’s get some rest, we both have work to do in the morning,”
Pulling out their shared futon, Souei cradled his lover close to his chest, his cool smooth body conforming perfectly to his against his oni’s.