He's Lättä (læt:æ), the correct translation in english is flatte. He's a derived meme from Apustaja, born in 2016 on ylilauta. He isn't retarded like Apustaja per se, but he's socially inept and unfeeling. Sometimes even malicious.
In a way, he is the antonym of Apustaja. Apu is stupid but means well, Lättä is calculative and chillingly intelligent but uses his gifts to toy with his victims.
Originally he was shitposted as a rapist, often associated with the term "penetrate" but that's mostly dead now. He still breaks into peoples houses and toys with them, knocking on their windows, leaving messages on their porches, dropping the mangled corpses of their pets down their chimneys.
The clown pepe is a derivative of him born on ylilauta too btw, but the current wave of honkposting was born on reddit and has nothing to do with ylilauta or actually funny frogposting.