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 life, which explains 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 cold to and completely insane towards those he holds some kind of deeper feelings. This could be the reason of Jin's obsession with killing Ragna and the behaviour he displays around him; being murderous and sarcastic while his voice sounds child like and displays a sort of twisted affection. Also this could be the reason why his anger and hatred towards Noel increased heavily after the Ikaruga War, taking in count at their first meeting Jin was polite and friendly towards Noel, despite being disturbed by her resemblance to Saya and during academy while they didn't get along like friends, Jin wasn't hostile towards her. Jin's attempt to kill Saya during the incident at the church seems to have been also triggered by Yukianesa's effect on Jin, turning his dislike for Saya into full hatred.

In Continuum Shift it is implied that Yukianesa is even a sentient being as Jin is show to exchange words with the swords, seemingly that the Nox Nyctores is talking to him and giving him orders that Jin doesn't want to follow and struggles to rebel against it. By the "dialogue" both have it can be 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 just messing him up, proved 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 threats 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, Jubei is the one who told Ragna this.

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 resulting in Jin not only increasing his power significantly to hold against Tsubaki (who had defeated him earlier in the story) but also to discover Phantom's location and dispell her magic, forcing both women to retreat. This is also shown when late 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 seems to almost disappear and think of saving Tsubaki and stopping threats for others than just satisfy his obsession with 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, possibly the reason why Jin now is training with him, to control better his blade and unleash his full power.