Raijinmaru is a capybara who lives at Tamakoma Branch and serves as the pet and constant companion of Yōtarō Rindō. In appearance she resembles a fairly ordinary capybara, with a large head, a big body, and short thin legs. Her fur is mostly light brown, with darker fur on the tip of her snout, her limbs, and the upper parts of her ears. Her true nature, however, is far from ordinary: she is a crown trigger, a class of exceptionally powerful trigger from the Neighborhood, and her assigned duty is to protect Yōtarō, the prince of the country of Aristera. Her name, which uses the characters for thunder god, hints at the power she keeps in reserve.

Aristera was one of the nations of the Neighborhood that had formed an alliance with the predecessor organization of Border, and it was destroyed roughly five years before the events of the story, forcing its royal family to take refuge in the human world, known as Miden. The country's mother trigger was inherited by Yōtarō's elder sister Ruka, and the crown trigger, which operates directly under the mother trigger, was given the order to guard the prince. Raijinmaru is that crown trigger, which is why she lives alongside the royal siblings at Tamakoma Branch even though she appears to be nothing more than a pet.

Little is known about Raijinmaru's inner personality, but she behaves as a very lazy, laid-back, and gluttonous capybara, spending most of her time resting and eating. In spite of this easygoing manner, she is capable of becoming very destructive should Yōtarō ever be in serious danger, acting decisively to eliminate any threat to him. Her motivation is simple: she carries out the protective duty assigned to her, and otherwise enjoys a relaxed life of food, rest, and the company of those around her.

Within the story, Raijinmaru mostly appears as a mascot-like resident of Tamakoma Branch, accompanying Yōtarō during daily life at the branch and on outings beyond it. The revelation that she is a crown trigger gives added weight to Yōtarō and Ruka's origins and explains the formidable power she holds when roused. In the anime special The Adventure of Yotaro, she travels with Yōtarō as he heads toward Shizuka City hoping to catch up to Konami.

Her central relationship is with Yōtarō, who is both her owner and the prince she is bound to protect, and the two are almost never seen apart. She is also tied to Ruka through the hierarchy of Aristera's triggers, since the crown trigger serves beneath the mother trigger that Ruka now carries. The other residents of Tamakoma Branch treat her as a familiar and harmless presence. The ability for Yōtarō to communicate with her comes from his own Side Effect, which allows him to talk with animals, rather than from any special power on her part.

Raijinmaru shows little personal growth over the course of the series, remaining a constant and largely unchanging presence. Her development instead lies in the gradual uncovering of her identity: what first appears to be an ordinary pet is revealed to be a high-ranking trigger with immense destructive potential, a reframing that gives new meaning to her constant watch over Yōtarō. Her principal ability is that stored destructive power, which surfaces only when her charge is truly threatened; in peaceful circumstances she displays no extraordinary capabilities beyond those of a normal capybara.