|Manga Debut||Chapter 0.2|
A Strong Soul (強靭な魂, Kyōjin'na Tamashī) is one of the various soul types in the world and is extremely rare.
Strong are so powerful that they are worth 99 souls. It is unknown what criteria is met to gain strong soul. In the manga, the appearance of the soul is larger then ordinary souls. In the anime, though it is not directly introduced, Strong Souls appear to be of a blue color like that of a human, though much larger along with a longer "tail".