Chelsea is Ruri's bodyguard/tutor, and accompanies her when escaping from the Underground. Chelsea controls powers related to gravity, and has a personality to match. She is very dedicated to her task of protecting Ruri, and would gladly lay down her life - in fact she spends much of the early episodes bashing Rumina on the head for being too familiar with Ruri. Chelsea's gravity-power is very strong, and she is a tough opponent. She always argues with Rumina and always gets into a fight with each other, however she later respects him and developed feelings for him.

Under the capital city of Tokyo, Japan, there exists a large, vast, and unknown world known as Underground. There, people known as Elemental Users exist; people who have the ability to control the elements: Fire, Water, Lightning, Magnetism, Freeze, etc. Meet Rumina Asagi and his best friend Ginnosuke Isuzu, two average high school freshmen who reside in Tokyo. When they meets Gravity User, Chelsea Rorec, and the Miko of Life, Ruri Sarasa, their whole life changes into one big adventure.
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(Source: Anime News Network)

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Sealed and forgotten below Tokyo, there is a world called Tokyo Underground. Bodyguard and element user Chelsea Rorec escapes the underground with the Maiden of Life, Ruri Sarasa. They take refuge in the home of Asagi Rumina but are soon attacked by members of the Company ordered to retrieve Ruri.