It is a manga about the lowest level demon lord and he has to fight the “hero” of his universe and he tries to call for help from the second or third strongest demon lord because she used to have a crush on him or something. But she’s too late.
The hero didn’t want to kill him but one of her party members stabbed him. Then he’s reborn and now he’s the hero’s best friend. He then makes it his mission to make her even stronger to defeat the demon lord faster so he can be lazy.