Dr. Xeno (Dr.ゼノ) is a scientist and a leader of an unknown group of survivors residing in the United States. He is the main antagonist of the New America City Arc.
"Either he's honest to a fault... or the world's greatest liar. It's not a problem, because I love both types!"
— Dr. Xeno, Dr. Stone, Chapter 151
Dr. Xeno is a young man in his mid twenties with long, light-colored hair that has been rather meticulously styled into a large pompadour in the front while slicked and spiky towards the back. His most notable facial features are the large colored markings over his face in the shape of an 'X', presumably to represent his name while also covering up his petrification scars. He has thick eyebrows and dark, serious eyes, and is most often seen with a smug, confident grin on his face.
In sharp contrast to the residents of the Kingdom of Science, he dresses quite elaborately and in modern clothes, donning a long black jacket with a high collar and various designs on the breast, with a light dress shirt and tie underneath. For leg wear, he wears fitted black trousers and shiny black dress shoes. He also wears stone claws on his gloved hands, similar to the clawed weapon used by Ibara.
Similar to Senku, Xeno is brilliant when it comes to science. He likes to give a lecture over scientific details, as noted by Stanley.
He is very observant, as he noted that Gen's group was skilled at facing the dangers of crossing the seas of the Stone World and that Gen may be lying to him after noticing he didn't fail his polygraph test.
He is somewhat eccentric, as he would invade Gen's personal space while asking for his identity in a menacing manner. According to Luna, Xeno would often state the word "Elegant", when something amazes him. This allowed Senku to remember him.
Xeno has proven to be condescending, he once stated in the case that he was sent back to the Stone Age that he will rule as a dictator over all those he saw as beneath him. Xeno belittled Magma in the call to the Perseus and requested to make contact with his fellow scientific equal. Xeno seems to underestimate the potential threat caused by Senku's group, as he labels them a "ragtag band of merry youths playing with science". He also has no qualms about starting a full-scale conflict with them, breaking contact the moment his conditions for the negotiation were rejected. Ryusui noted that judging by the behavior of his group so far, telling Xeno about the revival formula would spell doom for them since he is likely to make an army equipped with modern weapons to wipe them out.
Xeno is also fond of his protege Senku. After Luna relayed a silent message to Stanley that Senku was with the Japanese exploration group, he immediately ordered Stanley to not fire and quickly deduced Taiju was a proxy used to protect Senku. Unfortunately, his affection for Senku is ultimately outweighed by his desire for complete control of the Stone World. With brief hesitation, he ordered Stanley to shoot him anyway.
Skilled Scientist: As a former NASA Scientist, Dr. Xeno is a very capable science user. As it turns out, in the past, Xeno served as Senku's mentor. He was a NASA scientist prior to the petrification event. He was capable of identifying the tactics Senku's group used to bring down Stanley's aircraft. Senku acknowledged that Xeno's technological advancement was superior to his group's. It can be presumed that he is responsible for the construction of the fortress the Americans are using in San Francisco, the planes that were used to attack the group, and the various kinds of machinery they utilize for farming. The polygraph test used for Gen Asagiri's questioning is also likely to have been made by him. He also created a communications system to speak with his enemies or his colleagues. However, he wasn't able to come up with a revival formula for the petrified statues, something Senku came up very quickly after he was depetrified. From the recording of the transmissions he had his enemies, he was able to accurately calculate the age, height, and build of Senku . 
Skilled Leader: Dr. Xeno has been proven to be a capable leader, as he rallied his former NASA colleagues and a military soldier under his control after they depetrified.
Mental Resilience: Dr. Xeno has a great mentality, as he mentioned that he and his followers kept themselves awake for over thousands of years.
Linguistic Proficiency: Dr. Xeno has revealed himself to be bilingual, speaking both English and Japanese. However, he has stated that he is not very fluent in speaking Japanese.
One day at NASA, they received a strange email from a young boy called Senku Ishigami. While his colleagues were hesitant to respond, Xeno replied and tells Senku what he would need to conduct his experiment. Xeno continued to correspond with Senku about science. Sometime later, he was asked on what he would do if the world was sent back to the Stone Age and he responded that he would serve as a dictator over the less sophisticated. In response, Byakuya Ishigami laughed at this in good fun. Five years later, Byakuya announced his son Senku would visit and Xeno is shocked to learn the boy he taught was the son of his colleague and will be at NASA. Xeno found Senku while he admired a monument of a ship and approached by silently standing next to him, both obviously knowing each other. When an excited Byakuya greets his son and ask why he seems spaced out, Xeno is seen walking away.
Xeno began investigating the petrified birds and wondered about the cause. He met with the military unit led by Stanley Snyder at the NASA headquarters. Xeno explained his findings before a mysterious light appeared. Xeno along with the rest of the world was petrified for thousands of years. He maintained consciousness in that time.
On April 4, 5739, Xeno was freed and after using the platinum from Carlos' ring, constructed the fortress in San Francisco. Xeno then put his group, consisting of people from his meeting just before the Petrification under his employ.  
Stanley contacted Xeno about how the enemy took down his aircraft, while Xeno informed the former about how the formula was concocted. Stanley ends his lecture and concludes the group has a skilled science user like Xeno. 
Dr. Xeno is present in the fortress factory when Stanley brings Gen Asagiri in for questioning. Stanley says that it's no wonder Dr. Xeno is found in his factory as Gen is put onto something that looked like a polygraph test. Dr. Xeno introduced himself along with Stanley, before intently questioning Gen on who he is and where he came from. After the latter gives his name and some of his history, Xeno is fascinated to hear that the man comes from Japan, having taken to face the dangers of the stone world to reach America. After Gen describes his leader's behavior, Xeno makes a point that the stranger may either be honest to a fault or a very good liar. He doesn't mind however, saying that he is fond of both types. At the end, Xeno inquires on the leader scientist's name, to which Gen replies with "Dr. Taiju".
He eventually locates the frequency of the group's communication and contacts them in order to start negotiating with Taiju, who is advised by Senku on what to say. Xeno states that they have a clear technological advantage, having a factory that can produce mass amounts of ammunition. He makes them an offer to completely surrender and serve him since his manpower is lacking as he has no means to depetrify the statues. Xeno also mentions that only those who have been aware during all 3,700 years can be revived with nitric acid alone, thereby unintentionally telling the group he has no idea about the revival fluid. As Taiju innocently attempts to answer Xeno's question about how to make the miracle formula, he is stopped by Ryusui who informs him they will simply crush the group once they have the manpower to do so. Xeno's offer is rejected, and he promptly ends the call.
When Stanley decides to eliminate the science user of the Kingdom of Science, Xeno tells a protesting Gen if Stanley says that he can do it then it will get done. 
Xeno is later seen excited by ending this conflict by finding and eliminating the science user.
A short time later, he presumably got word of Stanley's plan and compliments the solution to simply snipe the leader.
Xeno later overheard Max's transmission and heard the word Senku. Recognizing the name as that of his former apprentice, he ordered Stanley not to shoot Taiju who he realized was a proxy to throw them off. 
Xeno then described Senku as his former apprentice. Repeating his words, that Taiju is not the enemy scientist, but Senku is. Stanley asks Xeno to identify Senku. Having a recording of the Perseus’ transmissions, while reminiscing about his past with Senku. Stanley complains that he can’t hear their voices, but Xeno says that he'll use Senku's voice to calculate his height. Xeno estimates his height to be about 5’8” and his age to be 19. After Stanley spots his target, he questions Xeno if he really wants his former ward dead. Thinking it over, Xeno realizes Senku will oppose his plans. He then gives Stanley consent to take the shot. 
Very soon, the Petrification Weapon is delivered to Xeno. Xeno listened to Gen describing it and though his followers are skeptical of Gen's words, Xeno plans to research the device. He and Dr. Brody continued their new project while having one of the soldiers prevent Gen from hearing.
Sometime later, he and Brody completed their project, a submarine used to attack the Perseus.
- Xeno is the American equivalent of Senku.
- He is a scientist like Senku.
- He revived on his own like Senku.
- He is responsible for the scientific weapons of his side.
- He uses a catchphrase "Elegant", which is like Senku's catchphrase "ten billion percent"
- They de-petrified on the same day.
- There are key differences between the pair.
- Xeno is much older than Senku.
- Xeno has never discovered the Stone Formula.
- Xeno spent more time on building up their weapons and buildings than Senku did.
- Xeno lacks Senku's morals and kind nature, best evidenced in Chapter 154 where both give their respective orders to the group. Senku wishes to capture the enemy scientist while Xeno wishes to eliminate the scientist in Senku's group in order to gain control of his group. He also seems to heavily look down upon those he considers of "lesser intellect" in contrast to Senku who (playful teasing aside) cares for his friends and values each of them and their respective abilities.
- Xeno likely did not conduct experiments with nitric acid after depetrifying, which would explain why he didn't discover the Stone Formula.