Princess Marenia Typhoonica is a merpony (mermaid pony) hailing from Equantica, an underwater kingdom located in the seas of Equestria, but the kingdom was momentarly banished to Desteny Islands by Discord back when he was pretty much still a chaos loving nut, where alot of things went wrong for both her and her kind until A Savior would fix everything and return them home. However, she has one flaw. After Captain Taiklar killed her father, and almost killed her mother, Marenia has/had grown to distrust humans without giving a second thought. She is like Twilight, but has no horn, and was once an Alicorn, as she was also once punished to look like a normal merpony after falling in love with the horse prince of Desteny islands, Prince Derek, by the far worse Dead Sea. But that would eventally be reversed by the forementioned savior again. She later on gets a baby sea dragon assistant named Urchin thanks to Oceanious feeling she needed the aid given she's got a lot of work piled on with aiding both Derek and her mother's kingdoms, and for the fact that she can be just as stressed out as Twilight. She is essentially the Equestrian variant of Ariel.
Marenia was born in Equantica as an alicorn foal. Marenia is much like Twilight, but was born in royalty with her mother, Queen Oceanious, and father, King Barrier Reef, along with her big brother and sister, Aquatatic and Aquatos. However, before she could become a princess, she needed to learn about magic and friendship, and know enough to actually become a princess. So, she practiced her magic skills and actually improved in less than 20 minutes. Then, her mother signed her up for her Magical School for Gifted Merponies. But unlike Twilight, she passed her entrance exam with great progress. Impressing her mother, she realized she could become a great princess, and gained her cutie mark, a trident.
When Marenia grows up, she continues to study magic until her mother decides it's time Marenia made some friends. Unlike Twilight, she accepted this, and decides to search Equantica. She finds a few friends in Equantica, like Young Yeller, a kelp farmer who supplies food for Equantica, Golden Speed, a quick-flippered merpony who claims to be the fastest in Equantica, Azul Raindrop, a hyperactive party-throwing blue merpony who makes all of Equantica smile, Watery Beauty, a regal merpony who is the royal designer for Equantica, and Gubby Caution, a shy merpony who cares for sea animals. They make great friends, and have started bonding for a long time.