M/F merpeople romance, royal and guard, villain is Jekyyll & Hyde style
it is an M/F romance based around fairytale type retellings including the first book with the villian as a Jekyll&Hyde. The heroine and hero are both merpeople that can transform from tail to 2 legs. the heroine is a royal and is going to a human school asnd lives in a dorm with a roommate. The hero is a royal guard that climbed the ranks very quickly at a young age since he was naturally gifted. He was assigned (self-assigned?) to protecting her while she is at school. She thinks he doesnt like her and he does secretly. They go to the water and to the underwater city/ palace area. There is barrier around the city that activates and deactivates and one of the times, the hero has to break through the barrier and mind-controls a whale to set off th sensor in another area. They also go to this water pond/ waterfall area in the underwater city and mate there "under the eyes of God" and there is a cave that they hide in because the royal family was held under seige and people were looking for them. One specific scene is he pushed her to swim ahead of him to escape and she stays and swims in the shallows waiting for him and when he shows up, they walk together back to her dorm and her dorm room is locked so he boosts her up through her dorm window onto her desk. This may be a combination of both the 1st and 2nd book in the series because the first book may be short or i just read them really fast?
it was a free kindle book i think and i read it like 10-15 years ago. the villain in this story was a jekyll and hyde breed? species? i really cannot remember anything else.
Thank you for any help and i can try and answer questions if you need more info.