Masamune Nakatsukasa is a character from the Soul Eater series, a member of the Nakatsukasa clan and the elder brother of Tsubaki Nakatsukasa. He appears in the story as a demon blade, a type of weapon that can transform into a katana. His physical appearance includes long black hair and gray eyes, and he is often seen wearing a cape.

In terms of background, Masamune was initially a well-mannered and caring older brother who had a close relationship with Tsubaki. However, his personality shifted dramatically after Tsubaki inherited the family's rare ability to transform into multiple weapon forms. Masamune believed that he was more worthy of this ability and felt that it should have passed to him. This resentment caused him to become bitter and power-hungry. He began to embrace a philosophy that only the strong deserve to survive, while the weak exist only to serve the strong. He adopted an archaic way of speaking and developed a fondness for writing haiku.

Driven by his bitterness and desire for power, Masamune left his clan and wandered the world as a rogue demon blade. He was unable to use his abilities on his own, so he would possess others, using them as wielders to collect souls and grow stronger. He played on the fears and weaknesses of those he possessed, eventually overpowering their souls and taking control of their bodies. His ultimate goal was to become a Kishin, a being of immense power born from madness and consumed souls.

In the story, Masamune serves as an antagonist who confronts his sister Tsubaki and her meister, Black Star. He represents a challenge that forces Tsubaki to confront her own insecurities and prove her worth as a weapon. The conflict takes place both physically and within the mental world, where Tsubaki faces her brother's resentment.

Key relationships center on his sister, Tsubaki. Masamune viewed her as undeserving of their family's legacy and resented her for it. However, during their battle, Tsubaki demonstrated that she truly cared for him despite everything. Moved by her sincerity and love, Masamune allowed himself to be defeated and absorbed by Tsubaki. This act marked the first soul Tsubaki successfully absorbed, granting her his katana form and all of his abilities. After this absorption, Masamune's consciousness resides deep within Tsubaki, appearing as a large deer in her mental landscape.

Masamune's notable abilities are centered on possession and shadow manipulation. He cannot attack directly without a wielder. His primary technique is Soul Possession, which allows him to overpower and control the soul of his user. Once in control, he can use Shadow Puppet, creating a controllable shadow from the possessed person for pincer attacks. From this, he can perform Puppet Strike, a long-range lunge thrust by folding the shadow onto the blade. Split Branches involves spikes emerging from the shadow's body at close range. Puppet's Rain restrains an enemy with multiple shadow puppets before stabbing them.

Masamune's development is marked by his shift from a caring brother to a resentful antagonist, and finally to a figure who finds resolution through his sister's compassion. His defeat and absorption allow Tsubaki to grow stronger and inherit his abilities, while his lingering presence within her serves as a reminder of their bond and his painful past.