He was told to put poison in the prince's food when he was a kid to weaken him. Nara only did that to help his sister (the sister would only get medical help when Nara does his duty, I think it was medical stuff but Im not 100% sure, had sth to do with his sister). During the process he changed his mind tho cause the bond between young Tasara and him was pretty strong already. So he ate the food containing poison himself (thats why he as a different fond when speaking, imitating the husky voice he got from taking the poison). Nara was the only person Tasara trusted and became friendly with so the betrayal rlly hurt Tasara allowing the "evil" spirit to take over his body more quickly. Im not sure where evil Tasara is rn tho, he is pretty fond of Nara cause he kinda unlocked him and bc Nara was the only one showing love/ protecting one of his human vessels.
He hates Nara because he was Tasara’s only friend and Nara poisoned his food for the sake of his sister. But Nara drank the poison for him which is why his voice is fucked up, he also tried to make it up by being kind to him.
Tasara heard the maid and Nara talk about poisoning him so he felt betrayed, tho Nara felt guilty and drank the poison, including in the food.
i haven't read the story yet, just skimmed it. what did nara do to tasara that made him hate him so much?