Oroku is the son of an ancient Higher Orc race and the Human Hero chosen by the Fairy King. His mom, the Hero was trained by the Slime Sage from Slime Tensei. His dad was the Demon Lord until he beat the Hero and fell in love. His mom is pretty wackadoo, and convinced him that his purpose in life is to build a harem of multiple races to further pass on the power of his father's race (the offspring inherit the characteristics of both parent's racial gifts). The first girl he met was Tita, she was captured outside the village's protective barrier while seeking herbs to heal her mother, and enslaved. Oroku rescued her, brought her home, and healed her mother.
Oroku's mother never taught him the finer aspects of romance (cuz she's wackadoo) so he just asks every girl he meets "Hey, wanna fuck?"
As per the norm of isekai manga post-2015; the humans are the shitheads of the world and all the "ancient evils" were already subjugated by the Slime Sage and MC's mom. So as far as we know, the Fairy King has chosen Oroku to be the next Hero (thus giving him control over Fairy spirits) because some "world calamity-level" shit is probably gonna be going down in the not-too-distant future.