This is the world of Speculum, a world with unique elements.

The first and most notorious one is the existence of Amiculus. In this world, whenever someone is born, their soul is divided, and a Amiculu is created. These beings can take many forms and are mainly divided into three groups: objects, plants, and animals. They accompany their person until the time of their death, and whatever happens to them reflects on the person like a mirror.

The second unique element is the existence of places called "Constructs." These places can take many forms, but they all house evil and twisted creatures. Some are even said to create a personal nightmare for those who enter them. If these places stand for too long, they eventually break apart, allowing the creatures inside to go on a rampage.

To combat this, an organization known as crystallium was created, with branches all over the world. They recruit those who are qualified enough to become Explorers. Explorers are individuals who draw power from their Amiculu to fight. They display a wide range of abilities. (For example, those with animal Amiculus can make them grow massive, those with plant ones can manipulate their vegetation offensively, and those with object ones possess the most varied powers.) Explorers are tasked with invading Constructs and eliminating all dark creatures inside before it’s too late.

Explorers are ranked from D to S. Constructs also follow the same ranking system, and only those with a matching rank are allowed to take on missions to clear them.

Besides these ranks, there is also a special rank called the Omega Rank. However, only five explorers in the entire world hold this rank, and your famous older sister is one of them. Omega-ranked Constructs are extremely rare, but it is said that they may even contain dark gods.

You also happen to be an Explorer (you can choose what your Amiculu will be animal, plant or object). However, as a D-rank, most people tend to look down on you—especially your sister.