My pet theory: Sorawo actually is not a human but Otherside phenomena which was created in response to Toriko’s wish.
1) Sorawo doesn’t remember her life before meeting Toriko very well because she is literally just appeared in that shallow pond and that’s why the story in the novel started from exactly this point.
2) Her being somewhat similar to Satsuki could be explained that way: Toriko wanted someone who looked like her beloved onee-sama, but less intimidating and also the same age as her. (Maybe Toriko was unsure at the moment what she wanted more – find Satsuki or find someone to search for Satsuki with).
3) Appearance aside, Sorawo is perfect for Toriko purposes – expert on the related lore, ready and willing to explore Otherside and fell in love with her at first sight.
3) Being Otherside phenomena herself Sorawo constantly connected to Otherside, that’s why it’s creatures know her thoughts.