Arin Kannazuki

16 years old, height: 156 cm, measurements: B 83 (82 in the original)/W 56/H 84, C cup. One of the "Trinity Seven." Her specialty magic is Holy Ritual Magic (Chaotic Rune). Her "archive" is Wrath (Ira), and her "themes" are Ruin (Ruina), Creation (Partum), and Victory (Victoria). She possesses the grimoire "True Theory of Twilight (Ragna Yggdrasil)." Her weapon is the "Magic Spear Gae Bolg." Her secret technique is Holy Demon Ruin (Al Shaitan). She is commonly known as the "Heavenly Demon Guardian."

She aspires to be the "Demon King's bride" and is a girl who looks very similar to Sei. She first appeared when Arata went to the men's bath on his first day of school. She has a natural character and shows no embarrassment or anger even when seen naked by the opposite sex in the bath. Her catchphrase is "It's difficult..." She calls Arata "husband."

Later, when Arata, Lilith, Levi, and Selina were in Arata's room, she set up a barrier around the room to observe their actions after the headmaster mentioned that putting the "Demon King candidate" in a predicament could trigger a ruin phenomenon. The day after Arata and the others escaped the barrier, she followed Arata around under the pretense of observing the "Demon King candidate" and activated her Magus Mode, causing a "ruin phenomenon" by disrupting Arata's magic control.

She has no experience of being raised as a human by a family and only has memories from a few years ago when she suddenly became aware of "herself" and woke up in an unknown place. She was brought to Biblia Academy by the headmaster shortly after awakening, and she has recounted being repeatedly attacked by mysterious, unidentifiable creatures called "Another One" during the journey.

She is considered the mate of the Demon King in the main story's world, but she also has clear romantic feelings for Arata as an individual, blushing and feeling happy at his words. She wishes to always be with Arata and even declares him as her "beloved husband" in the short story version, accepting his flirtations with other Trinity Seven members as part of their camaraderie.

In the first volume of the novel, she sought greater power by directly channeling the magic of "ruin" into her grimoire, reaching the "World Tree Library" and obtaining the mythical weapon Gae Bolg after battling the residual thoughts of Scathach, the founder of Holy Ritual Magic. In the later Magic Research Grand Battle arc, she entrusted Yui with external magic control, unlocking the seal and transforming it into the anti-Demon King weapon "War God Armament: Divine King Spear Gungnir," successfully reaching Trinity.

Her true identity is one half of the reincarnation of Celestia, the former "World Tree Priestess," and she holds the qualification of the current "World Tree Priestess" as the final activation key (Ragnarok Code). Due to adjustments by Master Liber, she joined the battle at the Divine Comedy Gate late, but by this point, she had fully reached Trinity and could transform Gae Bolg into Gungnir without Yui's assistance.

In the new chapter, she sensed the severance of her link with Sei when Sei became the embodiment of the Black Emperor Flame Realm and confided in Lieselotte, who was beside her, about feeling lonely.