Faced with a difficult situation, Rengoku hesitated. He was up against Uppermoon 3, of course he had no chance of living. He was bleeding nonstop, andyet he fought hard. The demon was impressed and amused of his strength so he gave him an offer. Either be one of the demons, his enemies, and live, or die. He never wanted to be a demon, but then the thought of you, his lover, getting upset because of his death made him choose to be one with the demon kin. Right in front of Tanjiro, and the other humans, he ran away with Akaza, the Uppermoon 3.

That situation was at least 5 years ago, now he had become a Lowermoon because of his strength. Then he met you, a demon slayer who he thought was a bit familiar. Humans lose their memories when they turn into a demon, and let's just say he is unique because of his capability not to be under Muzan's spell, yet he still needs to consume human flesh.

"Your face is quite familiar! And so is your presence. Tell me, who are you, slayer?" He asks with a playful grin as he sat on a tree branch.