Ice Sword: Yukianesa

Mucro Algesco: Yukianesa is a Nox Nyctores in the shape of a Japanese sword. This weapon was given to Jin Kisaragi by Saya during their childhood.

Information
In contrast with a real Japanese sword, Yukianesa does not actually have a blade. Its sheathed form is hinted to only have the hilt tightly locked by screw locks on the sides of the hilt. When it is drawn, the Nox Nyctores absorbs seithr and control water particles to both unseal the lock and generate a blade of permafrost that freezes everything it touches at frightening speed. According to Noel Vermillion, the ice Yukianesa creates is so powerful that it erodes all life.

Like all Nox Nyctores, Yukianesa alters the mental state of its owner. In Jin's case, his emotions are greatly amplified, breaking his usual cold persona and making him seem violent towards the people he is usually cold or indifferent to; and completely insane towards those he holds deeper feelings for. This could be the reason for Jin's obsession with killing Ragna the Bloodedge and the behavior he displays around him; his words become murderous and sarcastic while his voice sounds childlike and displays a sort of twisted affection. This also explains why Jin feels incredible hatred towards Noel Vermillion after the Ikaruga Civil War whereas during their first meeting and time at the academy, Jin was polite and friendly towards Noel (albeit disturbed by her resemblance to Saya).

It is also implied that Yukianesa is a sentient being as Jin is seen exchanging words with the sword, seeming as if the Nox Nyctores was talking to him and giving him orders that Jin did not want to follow and struggled to rebel against. By their "dialog", it is implied that Yukianesa has spoken to Jin in the past as well.

During his second confrontation with Jin, Ragna reveals to the former that Yukianesa is actually the one in control of Jin and the blade is manipulating him, proven in the first moments of their fight when Jin, against Yukianesa's will, tries to fight Ragna but cannot even draw the sword from its sheath. Ragna also seems to believe Yukianesa is a sentient being as he threatens the blade, saying: If you have an ass, you better watch it since Jin is capable of being the one to take control of it. Ragna also reveals that Yukianesa is acting as a power limiter on Jin and breaking free from it would make Jin strong enough to be his greatest enemy.

Trivia

 * "Yukianesa", "snow woman", is a Japanese fairy (more famously known as a Yuki-onna).