Yoshimori is the 22nd Kekkaishi of the Sumimura family. Throughout the series, Yoshimori is portrayed as a lazy, stubborn, headstrong individual who strives to protect his family and friends, especially Tokine, who he's had feelings for since childhood. When Yoshimori was much younger he often cried a lot and strongly dislikes being a Kekkaishi. Yoshimori's attitude greatly changed, however, after Tokine's arm was injured by an ayakashi that Yoshimori failed to eliminate. At the beginning of the series, he is often seen baking, an activity he often finds more important to him than his duty as a kekkaishi. He dreams of becoming a patissier, but is often chastised by his grandfather for this hobby.
Yoshimori's skills as a Kekkaishi differ from those of his counterpart, Tokine. Due to his rash and headstrong personality, Yoshimori has always fallen behind Tokine in terms of speed and accuracy. However, in terms of sheer power and stamina, Yoshimori excels. He is capable of creating extremely powerful and large kekkai, kekkai which Tokine is either incapable of forming or those which would quickly exhaust her. Throughout the series, observations had been made by Sumimura Shigemori and Sumimura Masamori that Yoshimori has an enormous wellspring of latent abilities in his kekkai. It has also been noted that Yoshimori is able to activate zekkai without any particular training beforehand. However, his zekkai could only be powered to its full potential in moments when his emotions overflow their boundaries.
SpoilerOnly three occasions had been shown with him using his fully powered zekkai in the current running of the series: once while fighting against Kaguro, while saving his brother Masamori from getting hit by a fatal attack, and after Hiura is severely injured. This state rejects everything he doesn't like including death. It is later revealed by Hazama that this is actually a different technique entirely, called Shinkai, which up until then he was unable to use unless a powerful being lent him the power to trigger it without the necessary training..
The death of Gen has also drastically changed his views, leading him to desire become more powerful. He sees Hiura in a similar state as Gen, and told Hiura that if he wants to kill someone, kill him first. Yoshimori didn't trust either at first but began to warm up to them as they fought together.
Yoshimori's power has recently undergone a significant change. At the behest of his grandfather, Yoshimori underwent special training with Shimano, a two-tailed cat spirit, to learn a technique that could untap the full potential of his power. After several weeks of training, Yoshimori learns to enter a mental state called musou, in which he is distanced from emotion and in a state of pure focus. While in musou, not only does Yoshimori's efficiency with power increases exponentially, he also gains speed and accuracy greater than that possessed by Tokine. Yoshimori is now training to enter "ultimate musou", a state in which not only will his full power be unlocked, but will enable him to summon his "landlord", a powerful guardian spirit which serves as an anchor to his heart/emotions. He is referred to as a sympathizer for Karasumori, he can communicate directly with the landlord as such.
As of chapter 266 Yoshimori has summoned his landlord, Shiguma, and can enter "Musou" state at will. In this state, Yoshimori not only can create seemingly indestructible kekkai, but also create large numbers of them with no visible signs of fatigue or strain. In chapter 273, Okuni reveals to Masamori that Yoshimori is an extremely rare human being that is compatible with Karasumori and if Karasumori chooses, Karasumori can share his massive 'spiritual reservoir' that has the power potential to destroy the world with Yoshimori.
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