"Holy Hearts!" is a manga series that blends elements of romance, comedy, and supernatural themes. The story centers around Riku, a high school student who unexpectedly becomes the guardian of a group of angelic beings known as Holy Hearts. These beings are tasked with maintaining balance between the human world and the celestial realm. Riku's life takes a dramatic turn when he discovers that he possesses a unique ability to communicate with and protect these angels, a power that sets him apart from ordinary humans.

The Holy Hearts, each representing a different virtue such as love, courage, and wisdom, are sent to Earth to combat malevolent forces that threaten both worlds. Riku's role as their guardian involves not only protecting them from harm but also helping them navigate the complexities of human emotions and relationships. The angels, despite their divine origins, exhibit human-like personalities and flaws, which adds depth to their interactions with Riku and the people around them.

As the series progresses, Riku forms close bonds with the Holy Hearts, particularly with the lead angel, Seraphina, who embodies the virtue of love. Their relationship evolves from one of mutual dependence to a deeper, more complex connection that challenges the boundaries between the human and celestial realms. The narrative explores themes of sacrifice, loyalty, and the struggle to maintain harmony in the face of adversity.

The plot is driven by a series of conflicts involving dark entities that seek to exploit the Holy Hearts' powers for their own gain. These antagonists, often depicted as fallen angels or corrupted spirits, pose significant threats to both Riku and the Holy Hearts, leading to intense battles and moments of emotional turmoil. The series balances these action-packed sequences with lighter, comedic moments, often stemming from the angels' attempts to adapt to human life and their misunderstandings of human customs.

Throughout the manga, the characters undergo significant development. Riku grows from a reluctant guardian to a determined protector, while the Holy Hearts learn to embrace their roles and the responsibilities that come with them. The interplay between the characters, combined with the overarching narrative of celestial conflict, creates a compelling and dynamic story that appeals to fans of both romance and supernatural genres.