Ōga

is a former Kara Inner.

Background
Ōga was initially created as the eighth Inner to serve as a research partner for Amado Sanzu's experiments. When his research on the human heart hit a dead end, Amado augmented Ōga's brain so she could assist him in unlocking its mysteries. When she attempted to research Jigen's heart, he considered it an act of treason, destroying part of her brain before she could flee. This injury caused her to no longer have a will to live, and her body began to break down shortly after.

Having learned from Jigen's example of the connection between desires and one's life-force, she began abducting people from the Thunder Rail, placing them under genjutsu experiments that would force them act on their desire to survive; all whilst secretly taking part in them in hopes that witnessing it would restore hers. The scale of her abductions drew the attention of the Five Great Countries' Joint Investigation Team, who began to investigate the disappearances.

Personality
Ōga is an eccentric, if not amoral woman. Following her brain injury, she found herself without a drive in life, or even the desire to live. Hoping to restore that desire, she was willing to abduct and manipulate several people to reach her goal, and is very methodical and intricate in her designs. Ōga considers herself more refined than other Kara Inners because she doesn't actually kill anyone, merely keeping them in suspended animation. Her outlook on life-and-death struggles was that people are always instinctively driven to save themselves, inevitably causing conflict with others. She found Boruto's desire to survive different from what she saw in others, which he attributed to seeing it as having dreams instead of desires. Boruto's unrelenting desire to save everyone, even Ōga after she revealed her plot, caused her to blush and sparked in her a dream she chose not to reveal.

While posing as Yatsume, she acted quite friendly, greeting Boruto after he saved Shamo from falling to his death. When Boruto leapt back down to the crumbling floor to rescue the others, Yatsume attempted to talk him out of it, worried about him dying. When Boruto nearly failed to save himself and Shamo, Yatsume climbed down the ladder and saved his life, showing her compassion despite them having just met each other. While willing to cooperate with Boruto, she does see the merit in Batta's more selfish approach, not dismissing it immediately. She was quite emotional when she saw how much everyone wanted to survive during the "Calculation" experiment.

Appearance
Ōga is a young girl of average height and fair complexion with big, red eyes. She has long, brown hair reaching her lower back. As an Inner, she had the Roman numeral of "Ⅷ" tattooed on her forehead and wears grey robes. While masquerading as "Yatsume", her hair is done up in an intricate style: she has ox horns on both sides with tight braids under them and two thick antennae hanging over her face. She wears a white bandanna to cover her Inner tattoo, a light blue, clipped-shut jacket, and black shorts.

While masquerading as an old man, she used both genjutsu and a puppet that appeared as a short, elderly man of heavily wrinkled, light complexion. It has a wide smile with several missing upper teeth, pupilless green eyes, scraggy white hair reaching his lower back with a long white beard. It wears brown cloths and a red cap with a yellow tongue and white wings on the side.

Abilities
As a former Inner of Kara whose body was modified with Scientific Ninja Tools by Amado, Ōga was regarded as being completely monstrous in terms of power and ability. However, only her brain was augmented, and she doesn't consider herself suited for combat. She is fairly strong, able to hold Boruto's and Shamo's combined weight with just one hand, and has quick reflexes and great agility, being able to rapidly climb down a ladder and grab Boruto's hand before he could fall to his death. She has a keen sense of smell, smelling blood on Rokuro.

Ōga is a skilled puppeteer, able to manipulate one disguised from a far. She has proficiency in Lightning Release, able to flow it into her weapons. She can use Yin Release to project life-like images of people from afar, and while keeping herself and her targets under suspended animation, she can create a seamless experience with many intricate scenarios, maintaining them through a scroll, whether her targets are together or apart in it. Should any of them realise they're in a genjutsu, she can quickly layer another one to deceive them into believing they have broken it.

Labyrinth Game Arc
Disguised as "Yatsume", she joined the Thunder Train with several passengers. Eventually, they woke up secluded in a room, and she asked Rokuro about himself, noticing he was bound by seals and cuffs, Anaguma warned her to stay away from him. Ōga introduced herself through a male puppet, claiming that they survived an accident and were exposed to a toxic gas. Ōga believed they had a subconscious ability to avoid death, and wished to test it. When one of the captives drew a death card, he killed him, stabbing him though his head with a puppet. Boruto attacked the puppet with Wind Release. Having made clear how serious his tests were, the first of five experiments began when a Kusa-nin angrily knocked over one of the scrolls, he accidentally activated the "Collapse" test, sending everyone to new location atop of a floor that hovered over a large chasm. Ōga explained that sections of the floor would quickly crumble away and they needed to escape to a higher, safe ledge. There, she introduced herself as Yatsume to Boruto, and the two made their way to safety. Yatsume however grew concerned when Boruto went back down to help other captives. When Boruto had to save Shamo and propelled himself back up, Yatsume reached to catch them. She stood by Boruto as he criticised Rokuro for sacrificing Anaguma and Kakesu to save himself. At the end of it, only 14 captives survived, and he congratulated them, allowing them to return and rest before the next test.

While waiting for the next experiment, Yatsume saw value in both Boruto's and Batta's approach to the situation, asking Rokuro his opinion. When Rokuro deduced that the Kusa-nin was actually Ōga's underling, he forced him into confessing it, resulting in his death. Ōga confirmed it indifferently when he announced the next experiment, Rokuro picking the "Light" test. The captives were presented with candles, and he explained the flames symbolised their lives, them dying if the flames went out. After providing them with lanterns to keep them and explaining how they could be passed around, he sent them off. When the group who decided to cooperate had to force open a gate which required both hands to open, Boruto trusted Yatsume with his candle while he helped open it. In the next room, she along the other shinobi helped protect the non-shinobi from a dust storm while they protected the entire group's candles. At the end of the experiment, he intended them to try stealing each other's candles during a time-consuming portion. While the group containing self-centred people resorted to stealing others' candles, Boruto's group where they cooperated, exploited the wording of his rules to create a longer, communal candle, resulting in no deaths in that group. He was impressed by their ingenuity.

Before the next experiment, Yatsume tried to comfort Batta, but was rebuffed, so instead shared what she dreamt of doing if she escaped, in her case changing her hair colour, prompting others to share their dreams. When she tried to get Batta to share hers, she noted Batta's use of "need" instead of "want" when talking about money. For the next experiment, "Pressure" was selected, which he found the harshest yet. When the experiment began, and the ceiling was triggered to come down on them, Yatsume injured herself attempting to stop it with her Lightning Release chains. When they managed to uncover alcoves on the walls, Boruto allowed Yatsume one spot, due to her injury and in gratitude of her saving him in the first experiment. While Boruto and others tried to break open a hidden space under the floor, Batta considered killing Yatsume to take her alcove, so Yatsume demanded to know the reason behind her death, Batta admitting she needed money to buy back her home island from rich resort developers. Boruto convinced Batta to help him, exposing a weak spot on the floor. The extra space allowed everyone to survive the experiment. Ōga congratulated Boruto, as everyone surviving it would allow him to make the following experiments more interesting.

Yatsume was among those who noted that Boruto and Batta were taking a long time returning. Ōga complained that Boruto and Batta were late for the selection of the next experiment, "Calculation", in which each person had a card with a number from 1 to 8 in it, and had to collect enough to sum 7 to go through a gate within two hours. Yatsume's was 4, and she met with Namua, refusing his 5 when he offered it, unwilling to take it from her friends. He showed her his conclusion that at most five people could survive the experiment, and listened to Namua talk about how he felt about his past. They were joined by Boruto and Shamo before finding Batta and Fugō's corpses. When Rokuro arrived and tried to kill everyone to reach Ōga's goal of only one survivor early, she was emotional by how much everyone wanted to live. After Rokuro's defeat, he revealed that Batta and Fugō were already dead when he claimed their cards, so Boruto shared with them Batta's suspicion of Kiseru. Yatsume and Shamo helped Namua through the gate while Boruto stayed behind to confront Kiseru.

Namua and Shamo discussed Boruto's attitude as Namua's condition worsened. Shamo went ahead to search for medical supplies, and returned with Rokuro, the three of them deciding to capture Kiseru, Namua holding the gate open for them. They joined Boruto in cornering Kiseru, but he escaped underground with his Fire Release. Chasing him, Shamo protected Yatsume with his Earth Release from falling debris in the caves. After splitting the group, Kiseru targeted Yatsume, resisting her Lightning Release, and retreating when Boruto caught up and Shamo attacked him with Earth Release. She was scared of Kiseru, and asked Boruto to stop him. Shamo checked on a scroll his attack made Kiseru drop, and they discover the nature of Kiseru's mission. Shamo went on ahead, while an injured Yatsume joined up with Rokuro, catching up just in time to see Kiseru stabbing Boruto.

When he woke up, Yatsume and Rokuro did as well. She retrieved a map and access card on the Kusa-nin that Rokuro had uncovered as Ōga's minion, and used it to search for an escape. She refused to hand them over to Rokuro multiple times, and when they were split from Boruto, Rokuro attacked her, taking them by force to search for an escape on his own. When she acted as if Rokuro's jutsu had infected her, Boruto refused to kill her, and from the pain of her attacks, realised they were still under genjutsu. Dropping the Yatsume facade, she revealed herself as the true Ōga, explaining who she was and what she wanted. Due to her brain damage which left her without a desire for survival, she wanted Boruto to break the genjutsu and free everyone, but as the construct collapsed around them, Boruto refused to give up on her. Reaching out to Boruto when he reached out to save her was enough to cause her to leave the genjutsu herself, where she proceeded to free everyone. She left before everyone came to, and left Shamo a letter addressed to Boruto, which he delivered. In it, she teased him about trying to guess the dream that sparked in her, and promised to visit him one day.

Trivia

 * means "lie on the side".
 * Her alias "Yatsume" could be a shortening of, which means "lamprey".