Both swords are sentient weapons, housing the souls of warrior spirits associated with Shinmeiryu and, in particular, the Aoyama family. Shinaijou houses the spirit of Hisato Aoyama, a powerful swordswoman who fell victim to a shapeshifting curse and was forced to serve an evil sorcerer for an unspecified number of years. Eventually, she was defeated in battle by Cris Nakakami, who freed her by slaying the creature she had been turned into. Her soul remained in the sword, which came into his possession, and has been passed down the Nakakami family line ever since. Kagetaku houses the spirit of Kazue Aoyama, Hisato's most trusted kunoichi. Prior to engaging the sorcerer, should the worst come to pass, Hisato asked Kazue to bind her spirit to the blade Kagetaku so that she might serve the rightful wielder of her master's blade.
Shinaijou possesses several special abilities due to the presence of Hisato's spirit. It was forged using an early version of Shinmeiryu's modern special forging technique, resulting in its ability to more easily channel a user's ki when performing the maneuvers of the school. The weapon itself also deals increased damage to demons and evil spirits. Its most notable and obvious ability is an empathy that can sense dishonesty, truth, love, and heartbreak in its vicinity, lending to its name. The weapon reacts differently to each of these stimuli.
Incidents of lying cause damage to the blade in proportion to the severity of the lie: small little white lies will produce a sound like someone striking the blade with a hammer but cause no actual damage, while a more potent lie will cause a crack to appear in the blade with an audible noise, and a lie of significant magnitude will cause the blade to shatter entirely. Speaking the truth around the blade, especially when it might be detrimental to the speaker, proportionately strengthen the blade and increase its sharpness. It should be noted, however, that the weapon seems to be able to selectively ignore these stimuli, particularly in combat when it would negatively impact the wielder, and therefore effectively functions as a lie detector outside of combat.
When exposed to love--most typically romantic love, though platonic love and events that demonstrate true companionship occasionally prompts the same response--the blade of the weapon emits a white light that soothes and comforts all who view it. If the presence of love is powerful enough, the light emitted by the blade can be visible even if the weapon is sheathed, and even raise to blinding levels. Conversely, when exposed to emotional pain, specifically heartbreak, and especially in circumstances when a pair who are meant to be together are kept apart for any reason, the blade weeps openly, with no apparent change in magnitude based on severity of the emotional pain as the other stimuli cause.
By comparison, Kagetaku's special abilities almost seem trivial. The blade of this wakizashi never dulls, regardless of use, and strikes with this blade seem more keen than they should be. However, the most important ability of this weapon is that the wielder can summon Kazue to perform tasks for him or her. There is no limit to what tasks Kazue can be assigned other than her physical ability to perform them, nor is there a restriction on where or when Kazue can be summoned. Hisato's spiritual energy is not high enough for her to be summoned, but she can manifest herself in magic-rich areas for a limited time.
Currently, the two blades are wielded by Seno Nakakami. He received Shinaijou from his parents after returning from his high school exchange program, and for a time wielded the katana alone as his primary weapon. It wasn't until after he and the other members of Kuro Arashi were awakened to their destinies that he was given Kagetaku to complete the set.