Neuro Wiki
Phantom Thief X
First Appearance Chapter 17
Voiced by Romi Paku
Age 20
Gender Born female, uses male pronouns
Status World-class thief
Sicks' altered clone
Known Relatives Sicks (father/clone)
Yako Katsuragi (half sister, anime only)
Seiichi Katsuragi (father: anime only)

X (怪盗X/サイ Kaitou Sai) is one of the main antagonists who was introduced early in the series, known to the world as the "Phantom Thief Sai", famous for compressing his murdered victims into "red boxes" and returning them to the scene of the crime. He is the first villain to pose a threat to Neuro (the past ones only being culprits in murder cases), determined to kill him because of his demon origins.

In the manga, his partner in crime is the highly talented "Plane Crasher Imina", also known as Ai after her encounter with X.

His powers involve transformation, super strength and invulnerability.



X usually takes the form of a young boy with silver hair in his appearances as "himself". He wears loose clothing and is usually barefooted. This isn't his true form however due to his inability to remember who he really was, and is only used for convenience. Later in the manga, X's original form is revealed to be a young girl with long brunette hair. Notably, X has taken a similar form previously when he first met Ai.


X is a charismatic, "evil idiot" villain, with a child-like way of speaking and mannerisms. He is selfish, care-free and indifferent, not being bothered on the slightest by the hundreds of murders he has committed during his villain career. He is a very reckless and unpredictable person, often changing from a calm state to a homicidal rage in a matter of seconds. Because of this, Ai stands out for being the only who can play the "straight-man" character around X without getting killed.

X is notable for being unable (or perhaps unwilling) to comprehend the value of human life, as he considers humans as his playthings whom he can kill any time he wants, whether it is to pose as someone else, or to search through their insides. To him, the subject of murder is as casual as talking about the weather.

After his modifications and retuning into XI, her personality has taken a complete overhaul. While X was a rather wild and unpredictable person, XI is a lot calmer, quieter and is completely loyal to Sicks. She shows little emotion, and mostly acts according to Sicks' orders only.


X, being unable to remember his true identity, goes around killing people throughout the series to see their insides as he feels he may be able to find a similar part he may be related to. He goes around killing several people ranging from tabloid paparazzis to politicians, hoping that their status and behavior may be related to his real identity. X gains a newfound interest in Neuro after discovering Neuro's similar abilities, and begins keeping an eye on his movements in hope that he may be able to see Neuro's insides one day and find out his own true identity.


X was originally an altered clone of Sicks, Experiment 11, and was born female 17 years prior to the current storyline. X's surrogate mother was killed after his birth. Shortly after the murder of Sasazuka's family (which was originally thought to be one of X's early murders), he would be released into public to gain more experience and information for the experiment. X slowly loses his memories with his repeated transformations however, and he eventually forgets everything from his past as Sicks' guinea pig and even his original gender. At some point in his life, X would encounter Ai on a flight to San Francisco and they would begin their alliance with the establishment of X's status as Phantom Thief X.

X's past is different in the anime series, where instead of being Sicks' clone he was the son of the "witch" Seiren and Seiichi Katsuragi, making him Yako’s half-brother.


X's transformation during their battle in the anime.

X gains HAL's Eye.

X's main power is transformation by the mutation of his cells, with which he has the ability to change his appearance in any way he wishes (even transforming into animals). He uses this power for many purposes, but mainly for disguising himself so that he can venture out in the public without being suspicious. During battle, X transforms his body parts into claws, extra feet and limbs or several other grotesque features. While not shown in the manga, X transforms into a mass of slime and completely engulfs his target to turn him into a red box by cutting, grinding and compressing them in his body. X appears to take the properties of what he transforms into, as shown when he turned his hand into a heavy concrete cube to squash a person in the anime. X's transformation abilities apparently improve throughout the series, as he notes in his second appearance he could not have transformed into a dog until recently. X's cells can be extracted and implanted into normal people to give them similar transformation and endurance powers as shown with the members of the New Bloodline.

Aside from transforming, he has super human strength and high endurance, meaning he is able to sustain heavy injuries, and occasionally even uses this to his advantage (such as when he pierces iron claws into his knuckles as weapons). Furthermore, because of this ability he can recover from serious injuries (such as gun shots) in a matter of seconds, although his recovery rate is directly proportional to the magnitude of the damage inflicted. The aforementioned ability makes X practically immortal.

Later in the series X gains HAL's Eye from a backup program of HAL, HAL II. He basically gains the Electronic Drug imprinted in his eyes with a modified version of HAL's logo. By staring directly into a person's eye and emitting a odd sound, X can control a normal person the same way the original drug could, though his version isn't as potent.

After having reached the final stage of his 'evolution' and turning into XI, he developed the power of reading people's mind by being able to read electric currents of the intracerebral flow.[1] XI uses this power to read the opponents' moves and can perfectly imitate the person in both personality and looks to the point.

While X is almost invincible, he has a few weaknesses which can also carry on to people using his cells. He can be temporarily disabled when he gets direct gunshots to his joints and he becomes severely weakened when he is electrocuted. His frequent transformations also causes X to have memory problems, as when he transforms he basically rearranges his body cells, including his brain cells, rendering him incapable of holding trivial information for more than one week (in the anime, his memories are “reset” every 6 months). Besides this, HAL's Eye is also rather flawed compared to the original Electronic Drug as it is 10 times weaker and wears off after a certain period of time or through repeated electrocution.

Plot Overview[]

X is first mentioned at the beginning of the Copycat Case, in which Neuro and Yako were asked by a father to help keep an eye on his son, who had been acting suspiciously recently. Neuro and Yako would soon find out that the son had been killing animals and stuffing them into boxes to replicate the crimes of his idol, X. During the reveal, Yako and Neuro successfully expose the son's crimes after he kills his grandmother but Neuro, sensing another outsider's presence, immediately calls out to the grandmother's corpse (in the anime, he calls out to the police officer at the scene) and tells her to stop pretending. The grandmother reveals herself to be X in disguise (whereas in the anime, he was disguised as the police officer) and promptly kills Copycat. X is arrested and he states his interest in Neuro before leaving. He would later kill the officers in the car on the way to the police station and disguise himself as Yako to get close to Neuro, where he learns that the latter is a demon and tries to kill him to see his insides, though Neuro calls his bluff and X escapes again.

Some time later, X would kill the Prime Minister and announce his interest in stealing a statue to the public. Neuro sees this as a challenge from X and goes to the Eshiya household with Yako to solve the case along with the Police Department. After exposing Kazuhige as the criminal, X makes his move and shoots Neuro in the head at point-blank range. Neuro barely avoids the shot by directing it to Kazuhige's direction but stays down for a while. X proceeds to kill Toshimitsu and injure Sasazuka, scaring most of the other witnesses away. He then breaks the statue to reveal the hidden necklace he had planned to steal, but finding that within it there were 'positive' feelings rather than the 'negative' ones X was looking forward to, he passes it to Yuka instead and escapes. Neuro would follow him to a construction site where they would have their first fight, in during which, Neuro reveals to X his body was gradually becoming more human; but X, refusing to accept this, grows even more determined to kill Neuro as soon as possible. He is defeated and crushed under the debris of the construction site however, but manages to survive. X takes different paths in the manga and anime version of the series after this.

In the Anime[]

Several episodes later without making an appearance (aside from a glimpse of him and Ai walking down the street after the HAL Arc, X kidnaps Yako to an underground temple in South America to confront Neuro once and for all (seeing how the deadline of 6 months was near its end), a place where he would supposedly hold the advantage (the place was said to be filled with an aura that was harmful for magical beings, such as Neuro). Yako manages to figure out X's mother was Seiren, a female witch-doctor, and Neuro proceeds to defeat him, making X collapse because of his injuries, and finally falling unconscious the moment his memories are reset. Yako then decides to take responsibility for X and brings him back to Japan; however, possibly because of the state of confusion and the amnesia in which he found himself the moment he woke up, he is shown to have escaped near the end of the finale.

In the Manga[]

X's true form.

X reappears after the HAL Arc, where he forms a small group consisting of himself, Ai, Leech and Kasai. Ai assists X in gaining HAL's Eye from HAL II, and he uses it on Leech to help him distract Neuro while he kidnaps Yako. Leech successfully diverts Neuro's attention and X manages to scare Yako unconscious by transforming into her father's murderer, inspector Takeda. He proceeds to bring Yako to his new hideout, and uses HAL's Eye on her to make her assist him in battle with Neuro when the time came. Three days later, after solving X’s cultivated puzzle, Neuro arrives at X's hideout where they begin their second fight. X transforms into Yako and has the real Yako fight alongside him, forcing Neuro to not attack during the fight, in fear he might kill the real Yako. Neuro manages to outwit and outmatch X however, and X is sent scurrying to the top of the building after his second defeat. X is cornered by Neuro, Yako and the Police Department when he reaches the top, but before he can get away on the helicopter Ai arrived in, Andrew Sixson kills Ai with a single shot to the temple. X crashes back on the building and Andrew, now revealed to be Sicks in disguise, captures X and flies off, leaving the rest of the witnesses confused and shocked.

After this, X returns to his life as a guinea pig for Sicks. He begins replanting his memories and X begins to remember his true origins at long last. His cells are also extracted during this period to be implanted into DR, Tierra, Sasha and Shura to enhance their capabilities. After Genuine’s defeat, Sicks decides to arrive to Japan alongside X, whom has by that time “completed his evolution” (regaining his original form and gender), and rechristens her as XI (Eleven), regarding her as the greatest weapon for the destruction of humanity.

X does not reappear with her original form until Sasazuka's rampage in town, where she disguises herself as Ishigaki to impale Sasazuka from the back, and to torture him by changing her face into those of Sasazuka’s deceased family members prior his death. XI then proceeds to hold Sasazuka still so that Sicks can deliver the final blow and kill him. Before Yako and Godai are able to reach Sasazuka, XI smashes a hole in the wall and escapes from the building they were in along with Sicks.

Sicks and XI would soon be ambushed by Ishigaki and the rest of the police force. While they manage to escape, they were visited by Neuro and Yako. They move to an abandoned amusement park near a forest and XI has a brief duel with Neuro, taking the forms of several past enemies. Sicks takes over after this and leaves Yako to XI. XI manages to corner Yako and tries to fool her into letting her guard down by transforming into Neuro, but Yako isn't tricked. She attempts to talk some sense into XI, telling her to remember Phantom Thief X and Ai. Hearing all this, XI begins having a breakdown. She seems to regain her senses however and returns to Sicks with Yako seemingly impaled through the torso. Sicks applauds XI for killing Yako and confidently taunts Neuro.

XI interrupts Sicks however and asks him to take a look at Yako's face. Sicks flinches when he sees Yako wasn't impaled at all and was simply knocked out. At this moment, XI transforms back into Phantom Thief X and claws through Sicks' chest, pulling out his heart. Sicks clearly becomes angry at the sudden betrayal and presses a button causing X to implode, followed by giving X a fatal wound with his sword. Sicks escapes after this and Neuro gives chase. Meanwhile, X, still barely alive, has a brief talk with Yako. Knowing she still wanted to see Sasazuka one last time if possible, he turns into Sasazuka and talks to her the way the real Sasazuka would have. Yako bursts into tears and embraces X, though X's injuries finally take their toll and X dies. Godai and Yako bury X's body after this and Yako plants a small tree on X's grave as a memorial.


  1. Matsui, Yūsei. "Chapter 180". Majin Tantei Nōgami Neuro. p. 5.