File:Cleo Sertori, (Anna Kay Faris), (main protagonist), (Semi-mer-Chipette, Semi-mer-Siren-Chipette, Semi-mer-Merette, Semi-mer-Siren-Merette, and Semi-mer-Power-Keeper-Chipette), (her normal High School outfit!).png

Backstory/Background:
Cleo Sertori, (Anna Kay Faris), is the overall  main   protagonist  of the H₂O/h₂o: Just Add Water Series. Before she  started  going to school, at South Coast High School, she'd  eventually  met Lewis McCartney, (Matthew Gray Gubler), when they were  five , during her first summer vacation or break. She got to  know  him very well, to the point where he'd met at her house for the very  first day of Elementary School. She would almost always sit with him at lunch, and play with him during recess. During the Holidays, she would almost always  surprise him with gifts that she knew he'd like. One time, she  gave  him his first laptop computer. Whenever his birthday came around, she would almost always  surprise him with the best  gift   ever . He would also do the  same   exact  thing to her. In class, she'd let him answer  most  of the questions ,  and  he'd done the same exact thing for her. Soon afterwards, they went to Middle School, and then , High School, at South Coast High. She would almost  always  sit with him at lunch, and talk about Zane Bennett, (Justin Jacob Long), or " bully  drama " as she calls it. One time, he'd  told  her about this sixteen - year - old girl named Emma Gilbert, (Amy Poehler), and she soon met her shortly  afterwards , at the lockers, shortly after Zane pulled a dirty locker prank  on her. The  two  of them have been great besties  forever  ever  since. She started sitting with her at lunch, and doing the same exact thing with Lewis, who decided to sit with them, too. During the Holidays, she would almost always  surprise  both  of them with  gifts  she knew they'd like. Whenever their birthdays came around, she would almost  always   surprise  them with the best  gift  ever. In class, she still tries to let both of them answer most of the questions, especially  since she knows that they're both  slightly - smarter than her. At  some  point during school, during lunch, Lewis told Cleo and Emma about this new sixteen - year - old girl named Rikki Chadwick, (Christina Applegate), and soon, Cleo and Emma met her. Lewis eventually met her, too. In the  first  episode, Metamorphosis, Cleo, Emma, and Rikki become merettes, under a Full  Moon ,  in  the Moon Pool, inside of an inactive  volcano. Cleo discovers that ten seconds  after touching or coming into  contact with water, she can transform  into  a Merette. The same exact thing happened to both Emma and Rikki. After Zane bullied her and Lewis, she discovered that she could  control  water. She discovered that Emma could freeze water, and Rikki could heat water, and  dry  off their tails.

Quotes:
"Emma, you were  really  murdering that!" — Cleo to Emma  about  her swimming,  and  Cleo's first lines.

"So cool ..." — Cleo to Emma  about  her swimming, and Cleo's second lines.