Mari Hayase is the protagonist of the anime Time Travel Girl. She is a second-year middle school student who lives with her mother Akira, a skilled patissier, and her younger sister Rika. Her father Eiji, a brilliant scientist, vanished without explanation three years before the story begins, leaving behind only a mysterious pendant known as the armillary compass. One day, Mari discovers an ancient book in her father’s study, and when the compass reacts to it, she is transported back in time. This event sets her on a mission to locate her missing father while also meeting eight of the most influential scientists and inventors in history, including William Gilbert, Benjamin Franklin, Alessandro Volta, Michael Faraday, Samuel Morse, Alexander Graham Bell, Heinrich Hertz, and Thomas Edison.

Mari is described as bright and friendly, with a determined and sometimes aggressive drive to accomplish her goals. She tries her hardest in everything she does, though she occasionally has a tendency to come up with ideas or solutions seemingly out of nowhere, reflecting a creative and spontaneous side. She is both curious and adventurous, always eager to learn about science and history, yet she also exhibits a cautious and responsible nature. She values security and guidance from those she trusts, and she is deeply loyal to her friends and family, often putting their needs before her own. This combination of boldness and reliability makes her a natural but sometimes anxious leader who grows more confident as her journey progresses.

Mari’s primary motivation is to find her father and understand the reason for his disappearance, but her travels also give her a broader appreciation for how scientific discoveries shape the world. She is supported by her best friend Waka Mizuki and Waka’s brother Shun, who accompany her on her time-slipping adventures. Together, they use the armillary compass to visit key moments in history. Through her interactions with the scientists she encounters, Mari learns about perseverance, ingenuity, and the human side of innovation. Over the course of the series, she develops from an ordinary middle school student into a more mature and empathetic young person, gaining a deeper understanding of work, friendship, and the importance of family. Her notable abilities include the power to travel through time using the compass, along with a resourceful and adaptable mindset that helps her navigate unfamiliar eras and solve problems on the fly.