This request refers to the popular young adult fantasy novel (translated in some regions as Poseidon Varisi ) by Anna Banks , which is the first installment of The Syrena Legacy trilogy. Series Overview
: The narrative alternates between Emma's first-person perspective and Galen's third-person perspective, offering insight into both human and Syrena worlds.
: Instead of traditional mermaids, the Syrena are powerful beings with shark-like fins who can transform into humans with legs to walk on land.
: The series explores the political tension and ancient laws governing two rival Syrena kingdoms: Poseidon and Triton . Main Characters Of Poseidon (The Syrena Legacy, #1) - Goodreads