Red Night Eve

Hana, a high-schooler that is known as the cutest girl in school, gets pulled into a supernatural event while on a beach trip with her friends. This event, called a "Red Night Eve" involves a transformed extremely massive shark, which targets Hana while she's swimming! Luckily, she's rescued by the siblings Yayako and Yuuto Kagimori, students from her school who know all about these occurrences, and who even have a support program AI named Acacia in their tablet to help them. Not long after, Hana and the siblings are involved in another event, this time involving her precious class pet parakeet being transformed into something monstrous. With all these supernatural events, Hana's life is never going to be the same!

(Source: MangaHelpers)