WARNING: The following contains spoilers for To Your Eternity Episode 4, “A Large Vessel,” now streaming on Crunchyroll.
When we first met March and Parona in Episode 2 of To Your Eternity, it was clear their relationship was more than just friendship. The two of them loved each other and treated each other as family, with March affectionately calling Parona "Nee-nee." When March was chosen as a sacrifice, nobody stood up for her, not even her parents. The only one who did was Parona, who was fueled by love -- and guilt for what happened in her own past.
Parona, like March, was once marked as an "untainted girl" to be sacrificed to Oniguma, and she likewise had a sister. The difference is, Parona's sister is now dead. She hid Parona away in the hollow of a large tree with a stuffed toy and enough supplies to last her for a few days, instructing her not to come out until she returns. A week later, Parona emerges and discovers that her sister had been chosen to be the sacrifice to Oniguma this year. All that was left of her was a foot. Parona's sister had saved her life.
The villagers shame Parona for running away from the ritual, accuse her of not being a woman -- because she didn't die for Oniguma -- and threaten to kick her out. But none of that even compares to her overwhelming guilt. It may be why we never see her face at the beginning of the episode -- Parona is too ashamed to face her. Her outstretched hand and her cry for her sister while escaping symbolizes her regret. Had she known the ritual's implications, things might have been different.
Parona has wrestled with this guilt for years and is trying to repay her sister by attempting to save March. She and March have grown to be more like a family now, and it's possible that March's own family took Parona in when she was ostracized. Parona absolutely does not want to lose another sister.
There's an interesting parallel between Parona's sister and March -- both of them had to make difficult choices for the sake of their loved ones. Parona was saved when her sister chose to be the sacrifice in her place; March understood that if she didn't go back to Hayase, her sister and friend would be the ones to die. So she protected them by choosing to not run away anymore. Parona wasn't able to save her sister but at the very least, she could try to save March.
Parona and Fushi both had someone who played a significant role in who they are today: Parona's sister and the boy, respectively. Both were nameless and died unfortunate deaths but they gave Fushi and Parona a purpose. Fushi, who had no inkling of who he even was, decided to see the world because that was the Nameless Boy's last wish. As for Parona, she wasn't sure why she had been the one to live. Until now.
Parona knows Ninannah's ritual has no basis in reality and the villagers' faith has been manipulated. Despite everything she went through from losing her sister to almost losing March, she still has faith in her people. The fake shaman warns her that Ninannah's traditions are too ingrained to ever change but Parona believes that if she were to bring back evidence of Oniguma's corpse, then girls like her sister and March would not have to die anymore.
While traveling to Yanome, Parona says humans need some kind of purpose to keep living. Although she had said this in relation to Fushi being immortal, Parona might have also been referring to herself. For years, she hadn't been able to bear seeing her sister's face again due to the sheer amount of guilt she carried. But now that Parona has found a way to stop the rituals from happening ever again, she can face her sister and tell her, without any regret, that she would "make good use" of the life she was given.