Veldora had been with Rimuru since around the beginning of his life. He saw the world through Rimuru's eyes for over 2 years before his release from [Infinite Prison]. Moreso, when Rimuru had been sent to the end of time by Yuuki, Veldora had already been inside Rimuru's [Imaginary Space] as punishment for trying to take Rimuru's energy and failing miserably during a crucial moment. He had waited along with Ciel untold eons for Rimuru to wake up, and had kept his faith in Rimuru that entire time.
Veldora knew quite a lot about Rimuru, probably more so than anyone else. He knew that the one thing Rimuru had always treasured above himself was the wellbeing of those he cared for. Veldora was the only person who knew for a fact that Rimuru's skill had grown an ego. He knew because of his conversations with it while he was trapped in Rimuru's [Stomach].
So when Veldora heard Rimuru, he immediately obliged, sharing all of their memories together across the connection, reminiscing with his brother over their adventures.
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Rimuru was ecstatic. Being told something, and experiencing it for yourself, are simply incomparable. His friends, which he did not know even existed until now, now reclaimed their position as the number one thing on Rimuru's mind. Overcome with joy, he ran towards Milim and Guy and hugged them with a goofy smile on his face. Guy's face was laden with bewilderment, while Milim's brain hurt too much to wonder what was going on, and decided to simply hug her bestie back.
The cascade of memories had halted once they reached the point where Rimuru left his companions to traverse the void. Thrust deep into thought, Rimuru decided that something must have happened to one of his closest friends to destabilize him that much. However, everyone was perfectly fine at that point. Left with no other answer, Rimuru concluded that his invisible companion in his mind that Veldora briefly mentioned must have dissapeared.
Rimuru also now realized that his loss of memories was not just due to time. If he had remembered everything after being sent to the end of time, thoe memories must have been preserved by this mysterious skill. And as it had disappeared, he was unable to keep his memories after all these years! Rimuru was quite proud of putting all of these puzzle pieces together so quickly after wondering for all these years, although his processing speed was higher than all comprehension, it still took him 6 seconds to come to this scenario.
Pondering what he could do, he kept on hitting the same roadblock. Nobody would be able to restrain him, so he couldn't really just come back here in good conscience after he decided to leave for the safety of others. But, he wanted to be with his friends. Oh god, he wanted it. After the events of the past few minutes, he would sooner rip himself apart than go back to his dimensional sightseeing.
An idea slowly started to take shape in Rimuru's mind. His existence had no equal, except himself. What if I went back in time and placed my roots in my own mind before even meeting Veldora? I could let events play out as they would, and the condition for my reawakening as the skill would simply be the disappearance of the skill that shared a brain with me in the past.
When awakened, I would act just the same as the skill that came before me, helping my own self, while making sure to keep others safe from me. This would even take care of the memory problem! This type of skill retained memories indefinitely. I would even get to spend the past 3,000-odd years with those that I care for above all else, as there would be no need to leave once I knew nobody close to me would be put in danger by my actions.
His internal monologue coming to a satisfactory conclusion, Rimuru walked over to Veldora, and hugged the storm dragon tight. Afraid to let go, tears of happiness were freely streaming down his face.
"Just a little while longer, Veldora" he said.