Ice Sword: Yukianesa

Mucro Algesco: Yukianesa has the shape of a katana. Continuum Shift states its blade is ice, and its power is manipulating the water in the atmosphere. According to Noel in Calamity Trigger, the ice Yukianesa creates erodes all life, explaining why Jin seems to freeze everything for a short amount of time during his Astral Heat. "Yukianesa", "snow woman", is a Japanese fairy (more famously known as a "Yukionna"). "Mucro" means the sharp tip of a sword, and "Algesco" means "to become cold". This weapon was given to Jin by Saya during their childhood.

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 and the behaviour he displays around him; his words become murderous and sarcastic while his voice sounds child-like and displays a sort of twisted affection. This also explains why Jin feels incredible hatred towards Noel after the Ikaruga 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).

In Continuum Shift, 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 didn't want to follow and struggled to rebel against. By their "dialogue", it is implied that Yukianesa has spoken to Jin in the past as well.

During his second confrontation with Jin in Continuum Shift, Ragna reveals to Jin 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, as told by Jubei to Ragna.

The one moment Jin is able to break free from Yukianesa's control was during his fight against Tsubaki. After arguing and having Tsubaki (while brainwashed and manipulated by Hazama) redeem Jin as his enemy, Jin is able to command and take control of Yukianesa. This results in not only increasing his power significantly hold against Tsubaki (who had defeated him earlier in the story), but also to discover Phantom's location and dispel her magic, forcing both women to retreat. This is also shown when Jin confronts Hazama in a one on one fight and Hazama retreats without second thoughts, stating openly that if the fight had continued, Jin would have won.

Regarding his personality, once free of Yukianesa's influence Jin becomes much more dedicated to what he believes is right. His obsession and selfishness seem to almost disappear and thoughts of saving Tsubaki and stopping threats to others take priority over his obsession with killing Ragna. While still cold and somewhat rude to others, he no longer seems to be violent and murderous. Even Jubei points out that Jin has grown, which is possibly the reason why he is training with him to better control his blade and unleash his full power.