MC (main character) was one of the four lackeys of Demon King. MC had the eye to see other's status, i.e. just like system. MC was the weakest of the four. But, among this four there was a girl that liked MC. MC and that girl had married just before they were killed by the hand of Heroes. MC died in the hand of Hero. Though MC was Demon King's Lackey, they were actually Demon King's slave. Even if they don't want to fight they have to fight for Demon King. Or, else they will die, because of slave contract. MC got reincarnate because of his System (Magic Eye). After Reincarnation he came in past and became childhood friend (who is a girl) of the Hero. But, during the time of blessing, he saw not only his friend but, he himself is a Hero. MC and his friend ran away from the village. They were exploring Dungeons to become strong. MC is Hero, and that's why MC wants to kill the Demon King, and take his revenge. And also save his past lover (who was also demon king's lackey and married mc just before death). I have read this much, and left it, just to take rest from reading and now forgot the title. Some one plz help.
Too lazy to look up the title but it's a Toika/toycar novel, like EER. Edit: couldn't help myself ; I Reincarnated For Nothing Although i feel some minor differences, i.e. he didn't become female and he didn't set out to kill the demon king from the start.