Subject Zero (dub)
Yu-Gi-Oh! ZEXAL episode 13: "The Number Hunter, Part 1"
Hart Tenjo, known as Haruto Tenjo (天城ハルト, Tenjō Haruto) in the Japanese version, is Kite Tenjo's younger brother and younger son of Dr. Faker. He possesses abilities from the Barian World, that Dr. Faker abuses in order to accomplish his plan of destroying the Astral World, and collecting the "Number" cards. He holds a strong connection to Yuma Tsukumo and Astral.
While under the influence of Vector's Barian powers, Hart suffers from a split personality: that of a sadistic nature with a desire to destroy the Astral World, as seen when he's laughing at its inhabitants' suffering, saying that he "needs more screams" (in the dub he says he needs more energy), as that is what comforts him. When he's not using his powers, he tends to act quite vulnerable, usually staying in bed, or looking down at Heartland City from the Heartland Tower, wondering why he cannot enjoy laughter like the people who attend the Heartland Fairground.
Kite had stated that Hart has not been his usual self as in the past, as he used to enjoy laughter and was playful and talkative and was very kind.
During the Duel when Dr. Faker is possessed by Vector, he was shown to deeply care for Kite and Yuma, as he could not bear to see them in pain and shedding tears. After the Duel with Vector, Hart was fully rid of the Barian's influence (with the blank stare and bags under his eyes gone) and his personality reverted to the way it was in the past.
Hart possesses Barian powers that can create paths to alternate realities and teleport himself to another location, especially when he is worried with his brother. Also, he is able to transfer his power to others, like he did with Kaito, in the form of Neo Galaxy-Eyes Photon Dragon. Moreover, his powers allows him to see Astral and even hear him.
Although he is fragile, his dub name is the German word for "tough" or "strong".
Hart was born very frail and sickly, and due to this, his father, Dr. Faker went to great lengths to save him. He eventually made a deal with Vector of the Barian World, who sustained Hart's life in exchange for Faker finding a way to collect the Numbers and destroy the Astral World.
In the past, Hart lived with his brother Kite and Orbital 7 in a small, wooden villa where they would play all day whilst Hart drank his favorite drink, hot chocolate. This happy life was however, disrupted due to the presence of Hart's "Dark Energy". Mr. Heartland appeared before Hart and his brother, and told Kite that Hart's powers are useful for the world and that Hart must go with him per Dr. Faker's orders.
Some time later, Hart was visited by his brother at Heartland Tower, but he couldn't remember Kite. Angered, Kite demanded from Mr. Heartland to know what he had done to his brother. The latter answered him Hart is able to communicate with the spirits of the Astral World. In order to save their own world, they must destroy the Astral World, and Hart had been chosen for that purpose. As Hart's soul had already left, the only way to heal Hart is the gathering of one hundred "Number" cards. Upon hearing that, Kite agreed to become a Numbers Hunter.
During one night at Heartland, Kite tried to escape with Hart. As they were hiding from Heartland Police, Kite gave him his favorite candy, caramel to cheer him up. However, they were found soon. Kite was subdued, while Hart was taken away from him. Seeing his brother being restrained, Hart used his powers to knock the police who were guiding him off of the tower. But they were both captured once more.
After hunting the "Numbers" of an unlucky Duelist and informing Mr. Heartland about his capture, Kite returned to Heartland Tower in order to check up on Hart. He found Hart stood at the window of the tower, staring blankly down at the people enjoying the Heartland Fairground and asks Kite why they are laughing as he doesn't require laughter, only screams. This Kite displeased, but Hart asked if he's normal, with Kite responding that he is simply in a bad dream and that he will one day wake him up from it.
During Kite's Duel with Yuma Tsukumo, Hart was ordered to send the trash collected by the litterbots into the Astral World again. This caused him intense pain and he collapsed, calling to his brother. Orbital 7 monitors Hart's situation and informs Kite, causing him to cancel his Duel with Yuma in order to run to his brother's aid, saving Yuma and Astral from not only losing their "Numbers", but also their lives.
After his trauma, Hart spent a number of days in bed. To keep Hart relaxed, Kite stays at his bedside, making sure that he is comfortable in every aspect, even to the extent that he has a holographic scenery of their old house surround Hart's room. Despite being ill, Mr. Heartland interrupted and stated that Hart must use his powers. Despite Kite's protests, Hart said that he will use his powers as they are causing good for the world, which surprised Kite. Hart then left with Mr. Heartland, who tells Kite to continue collecting the "Numbers" if he wants to save Hart in time, and the two then disappear through a portal. Some time later, Hart was seen sleeping with Kite watching over him.
World Duel Carnival
Later on, Hart sees Kite exhausted from collecting the "Numbers" to heal him, and after blowing part of his room, snuck away from the Heartland Tower. Whilst walking around Heartland City, Hart meets Yuma, Astral and Tori. Hart revealed that he can see Astral, who in turn wants to know more about him. Moved by Astral's wish, Yuma took him to his home, but in the way, they find Dextra and Nistro, who were searching for him. Hart then created a wave of energy and fled with Yuma and Tori. After Yuma agreed to take him to Kite,a helicopter (which was in fact piloted by Quinton) was about to land. As a result, Hart unleashed his dark energy, but Yuma calms him down with a caramel. Quinton then came out and uses his crest to take control of Hart, whom goes on the helicopter with him, and they leave.
Yuma and Kite then joined forces to go rescue him, breaking into an abandoned Gallery, where they met and confronted Quattro and Trey. While the four Duel, Vetrix began a ritual to extract Hart's powers.
While Kite is fighting, at the moment where he's about to lose, Hart resonated with his brother and, bearing in mind that Kite has always been there for him to protect him, Hart decides to fight to protect his own brother as well. As a result, Hart's remaining power was transferred to Kite, allowing him to Xyz Summon "Neo Galaxy-Eyes Photon Dragon", forcing the ritual to end, and leaving Vetrix with the rest of his power and memories. Hart had then been left in critical condition, and was rushed to the hospital.
World Duel Carnival Finals
While looking for his Deck, Yuma and Astral found Hart resting on top of the Heartland Tower. Due to his condition and known situation, Yuma cried for Hart only for him and Astral to be transported into a vision where they meet with Hart in the villa he lived in. In his consciousness, Hart said he knew they would come, and requested Yuma and Astral to save his brother.
During Kite's Duel with Vetrix, when Vetrix revealed Hart's face under the mask, he is shown on the big screen, still unconscious. Even so, he squirms in pain when Vetrix pinches his cheek, as the ritual had connected him to Vetrix.. Yuma and Orbital 7 rushed to Heartland Tower to save him, as Vetrix hacked the system and locked the door, preventing Faker's guards from getting in. Yuma and Astral entered Hart's consciousnesses again, where they met Vetrix, who took the form of a shadowy dragon. Yuma slew it with "Number 39: Utopia", severing Hart's connection to Vetrix and allowing Kite to continue the Duel, but Kite ultimately lost. When Vetrix attacked again, Hart appeared and protected him with an energy field, though Kite still fell comatose after losing his "Numbers".
When Vetrix released all the souls of his victims, Hart is shown to be waking up from his coma along with Kite. However, Dr. Faker took Hart and used him to power the Sphere Field Cannon. Kite teamed up with Yuma and Shark and attempted to defeat Faker and save Hart. Faker eventually revealed the truth of his deal with Vector, saying that Hart would not have survived up to that point without the power of the Barian World. Hart managed to use his powers to free Astral from the Sphere Field, letting him merge with Yuma and win the Duel, saving Faker from Vector's possession. After Fakers defeat, he alongside his father watched the Duel between Yuma and Kite.
Yu-Gi-Oh! ZEXAL II
After hearing that Yuma was hospitalized, Hart admonished Kite about visiting Yuma in the hospital instead of looking over data about Mizar. When Kite seemed to ignored him, he decided to go on his own until Orbital 7 warned Kite on how it was time to visit Yuma like he planned too, causing Hart to smile. On the night before Kite goes to the Barian World to help Yuma, Kite informed Hart of his mission, causing Hart apologized and blamed himself for causing Kite to go on a dangerous mission on his behalf. However, Kite insisted he going of his own choice and promised Hart that he will to return soon.
During the Barians' invasion of Earth, Dr. Faker and Hart watched Kite's and Mizar's Duel on the moon from their laboratory. As Kite was about to die, he comforted his saddened family by saying he had no regrets and told Hart to care of their father before the screens blacked out. After the fusion of the Barian World and Earth continued, Faker and Hart used Heartland Tower to house survivors who did not obtain a "Fake Number" and get sacrificed to the Barian World. As most of the people started to lose hope, Hart urged not to because Yuma was fighting for them.
After Nash was defeated by Yuma and saved the Earth, Hart went with Caswell Francis, Cathy Katherine, and Lillybot to congratulation Yuma's success. However, Yuma regretfully informed to Hart that he couldn't saved Kite. Some time after Yuma's and Astral's Duel, Hart started assisting with his father, Vetrix, Quinton and a revived Kite to research parallel worlds.
Kite loves his little brother Hart very much and he claims Hart is "everything" to him. In their childhood, Kite and Hart lived together mostly by themselves and would play every day, from catching butterflies to playing hide-and-seek. Kite would do anything it took to protect Hart, even at the cost of his own life, pushing himself to hunt "Numbers" on his own and disregarding the dangers of his Photon Transformation. Whenever Hart was in danger or pain, Kite underwent great panic and stress, to the point of not thinking clearly as his anger and worries took over.  
While Kite is normally cold and detached from others, Hart brings out Kite's gentle and caring side. Since Hart's "sickness" took a toll on him, Kite often tried to make him feel better, usually having limited success. Kite also tried to spare Hart from being concerned about him, putting on a forced smile and assuring him that he's alright. Although he goes along with Kite's words, Hart can see through him and understands what Kite was doing for him. If he was able to, Hart would try to protect Kite as best as he can within his powers. 
Having seen Hart's picture before, Yuma first met Hart by seeing him walking in the middle of a busy street and recognized him as Kite's brother. When an upcoming truck was about to run Hart over, Yuma tried to protect him. After Hart had blown a huge hole in the truck, Yuma kindly let Hart come home with them and let him stay for the night. Yuma was also happy when Hart finally started to eat something and enjoyed his time at dinner. At night, Yuma let Hart sleep in his hammock and covered him with a blanket.
While Hart was having a power surge, Yuma ran into the explosion and hugged Hart, telling him "you're not alone" and promised to bring him to his brother. Also, Yuma helped Kite rescue Hart from the Vetrix Family, and showed great concern when Hart was found unconscious. Yuma seems to have grown close to Hart; while speaking to him in his mind, Hart asked Yuma and Astral to save his brother, Kite. When Vetrix used his connection to Hart to hurt Kite, Yuma rushed to Heartland Tower to save him and severed his link to Vetrix. Likewise, Hart also cares deeply about Yuma as he is always doing things for his sake. Hart is also grateful towards Yuma for helping his brother overcome his guilt.  When Hart heard that Yuma was injured, Hart was very concerned about him and wanted to visit him at the hospital, even wanting to see Yuma on his own if Kite didn't go with him.
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Yu-Gi-Oh! ZEXAL episode 40: "About Hart"
- ↑ 2.0 2.1 2.2 2.3 2.4 Yu-Gi-Oh! ZEXAL episode 22: "The Shark Hunter"
- ↑ Yu-Gi-Oh! ZEXAL episode 55: "Portal of Doom"
- ↑ 4.0 4.1 4.2 Yu-Gi-Oh! ZEXAL episode 13: "The Number Hunter, Part 1"
- ↑ 5.0 5.1 5.2 5.3 5.4 Yu-Gi-Oh! ZEXAL episode 71: "A Trio's Challenge: Part III"
- ↑ 6.0 6.1 6.2 Yu-Gi-Oh! ZEXAL episode 72: "Kite's Plight: Part 1"
- ↑ 7.0 7.1 7.2 Yu-Gi-Oh! ZEXAL episode 14: "The Number Hunter, Part 2"
- ↑ 8.0 8.1 8.2 8.3 Yu-Gi-Oh! ZEXAL episode 43: "The Dragon Awakens"
- ↑ 9.0 9.1 Yu-Gi-Oh! ZEXAL episode 70: "A Trio's Challenge: Part II"
- ↑ Yu-Gi-Oh! ZEXAL Rank 8: "Galaxy-Eyes Photon Dragon!!"
- ↑ Yu-Gi-Oh! ZEXAL Rank 9: "Kaito's True Skill!!"
- ↑ Yu-Gi-Oh! ZEXAL episode 25: "Frozen in Time"
- ↑ 13.0 13.1 Yu-Gi-Oh! ZEXAL episode 41: "Losing Hart"
- ↑ Yu-Gi-Oh! ZEXAL episode 44: "Rock and a Hard Place"
- ↑ 15.0 15.1 Yu-Gi-Oh! ZEXAL episode 60: "Doctor Visit"
- ↑ 16.0 16.1 16.2 Yu-Gi-Oh! ZEXAL episode 63: "The Hart of the Matter"
- ↑ 17.0 17.1 17.2 Yu-Gi-Oh! ZEXAL episode 64: "Change of Hart"
- ↑ Yu-Gi-Oh! ZEXAL episode 67: "Sphere of Fear: Part 3"
- ↑ Yu-Gi-Oh! ZEXAL episode 68: "The Countdown Begins"
- ↑ Yu-Gi-Oh! ZEXAL episode 69: "A Trio's Challenge: Part 1"
- ↑ 21.0 21.1 Yu-Gi-Oh! ZEXAL episode 73: "Kite's Plight: Part 2"
- ↑ 22.0 22.1 Yu-Gi-Oh! ZEXAL episode 84: "Playing Defense"
- ↑ Yu-Gi-Oh! ZEXAL episode 95: "The Search for Shadows"
- ↑ Yu-Gi-Oh! ZEXAL episode 134: "Dragon Strife, Part 1"
- ↑ Yu-Gi-Oh! ZEXAL episode 135: "Dragon Strife, Part 2"
- ↑ Yu-Gi-Oh! ZEXAL episode 142: "The Battle of Three Worlds"
- ↑ Yu-Gi-Oh! ZEXAL episode 144: "Last First Duel"
- ↑ Yu-Gi-Oh! ZEXAL episode 146: "Forever ZEXAL"
- ↑ Yu-Gi-Oh! ZEXAL episode 50: "Party Panic"
- ↑ Yu-Gi-Oh! ZEXAL episode 42: "A Dubious Duo"
- ↑ Yu-Gi-Oh! ZEXAL episode 44: "Rock and a Hard Place"
- ↑ Yu-Gi-Oh! ZEXAL episode 56: "Cosmic Chaos"