Takumi has a large family, which consists of two sisters, a brother, and Corrin. He was raised by King Sumeragi (though Takumi was quite young when Sumeragi was killed) and Queen Mikoto. He has a younger sister, Sakura, and an older sister, Hinoka. His older brother is Ryoma.
Of all his siblings, Takumi seems to get along best with Sakura. They are able to chat with each other freely, and share many of the same opinions (such as disliking banquets and socializing). When the two were younger, they would frequently play together. Orochi's support reveals that Takumi and Sakura would sometimes be found holding court with some of their favorite dolls. The fact that he had dolls is embarrassing for Takumi now, but he enjoyed his youth playing with Sakura, and still enjoys spending time with her.
One reason why they get along so well could be because Sakura has such a gentle personality. Takumi is character who is quick to anger or become frustrated when critiqued or goaded, and Sakura is a character who is least likely to cause trouble for another. She is very kind and considerate of the feeling of people around her. Takumi can spend time with Sakura without constant worry of ridicule or even jabs of teasing.
Hinoka and Ryoma have a slightly different relationship with Takumi when compared with Sakura. While Takumi is more carefree spending his time with Sakura, he is much more serious when dealing with his older siblings. He constantly seeks their approval, and works extremely hard to gain it. He respects his older siblings greatly, and strives to be like them (especially Ryoma, who he desires to eventually beat in combat). But sometimes Takumi's quick-to-anger personality, along with his perceived lack of worth can cause problems.
In Hinoka's support for example, the two had been ambushed and Takumi received a leg injury. He told Hinoka to leave him, and to escape on her own. Hinoka of course, had no intention of leaving her little brother by himself with enemies around. At her refusal to go, Takumi exclaims that she should stop pretending to care about him, as he was a burden and everyone knew it. Hinoka is horrified and upset that Takumi would say such things about himself. This results in a fight between the two, centered around Takumi's perceived lack of self worth. They are eventually rescued and their issues are resolved with apologies, but Takumi's feelings of worthlessness about himself led to them fighting in the first place. Hinoka loves her little brother more than anything, and admits that if anything happened to him, it would break her.
Similarly, but not as extreme is Ryoma's support. Ryoma often spends time training with Takumi. Takumi greatly desires strength, so that he may serve at his brother's side in combat. Takumi very briefly quarrels with Ryoma when he expresses a strong desire to earn a place at his side (even though his birthright grants him this privilege). Takumi in this case, quickly had become frustrated with his inability to beat Ryoma with a sword. Ryoma explained that these things take time, but once Takumi is upset about something he carries it for a while.
Eventually the fight is resolved, and gives us a glimpse into another aspect of their relationship: admiration. Takumi values Ryoma's approval and opinion more than any other in the world. As a result of this, Takumi seriously begins taking up the study of government at his brothers suggestion in order to become a counselor once the war ended. Ryoma is very much the big brother type. He is stern, but encouraging towards his little brother, and always has his best interests in mind. He inspires Takumi not only to do his very best, but to expand his way of thought and learn as much as possible.
Takumi's relationship with Corrin (the avatar) depends greatly on the path that the game takes. In Birthright for example, Takumi is eventually comes to trust and love Corrin just like all of his siblings. They are able to joke and be friendly as if Corrin had never been missing in the first place. Though there are some bumps along the way, as Takumi is a very suspicious individual. Takumi even teaches Corrin how to use the bow in his supports. In Conquest, poor Takumi takes a much darker path and spends much of the game despising Corrin. It is revealed in the end though, that he had been possessed and something had been manipulating his darkest fears and feelings of rejection. He comes to accept Corrin in the end, and admits that he had desperately wanted to be a family with Corrin. As Corrin destroys Takumi's possessed body, he offers thanks for setting him free.
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