Clues to Raven's secret were present, even before you start to get to know her.
When Gaius gives Micah a tutorial about caring for befriended monsters, he remarks that Micah has a way with monsters, just like Raven. Micah and Raven are both half-monster.
Think about the bachelorettes who do not have a blood relative in the game: Daria, Kuruna, Pia and Raven. All are half or non-human.
Raven is one of six characters with pointed ears and responds to the Strange Scent cologne. None of the others are human. Gaius and Zaid are dwarves, Kuruna and Ondorous are Univir and Daria is an elf.
Karina often waits at Micah's house for her requests, and even stays overnight for one. At first glance, it's odd that she'd make the effort to visit Micah's house, considering her passive personality. However, since her arc focuses on her difficult relationship with Hazel, it's not a stretch to think that she's intentionally avoiding her mother by waiting for Micah at his house.