It's still really difficult to convey, because English is in fact not set up to be able to handle a direct interface between concepts like what he's used to. Plants aren't good at sequential reasoning and they are extra not good at abstract concepts but they are excellent at very precisely delineating the way things are alike or unalike. He can't be this precise, here, so he has to try to make do with various comparisons, and whenever the set of existing comparisons doesn't cover something or Yvette asks a question that implies some inference that isn't valid, he has a new comparison to point to.
There exists a person who was named Nai. There are some things that are always true of this person. This person is introverted, has a systematising way of dealing with problems, is very dutiful and diligent, has a strong sense of right and wrong, wishes to help those who can't help themselves. And those ways that person is can be honed and redirected in many different ways. Nai's relationship with his self-concept is somewhat skew to how most humans do it; he's immortal, and it would be absurd to assume he is just going to be the same person forever, "forever" is just too long a time for someone to stay the same. So whatever person he is at a given time is, in a certain sense, an "implementation" of that kernel of personhood. There is a Nai-template, and there are many Nais that would fit that template.
The Nai he is right now is... not a Nai he likes very much. He liked the Nai he was aboard the ship a lot more than he likes the current Nai. But he built the current Nai for a reason, he created this implementation of his self because that is what he needed to be. He needed to be a blade, sharp and cold and uncaring and most of all singlemindedly focused on his objectives. And while he is being this Nai, people have called him Knives. The Nai he currently is is a weapon, is a blade, is knives. But that is only one version of all Nais he could probably be, and it's a version of him that he will get rid of as soon as he can. He doesn't want to be a blade. He doesn't want to monomaniacally focus on his one goal to the exclusion of all others, to cut through everything extraneous and unnecessary and good so that he can get what he wants, what he needs. The Nai he is, right now, is a Nai he needs to be, as a tool to save his sisters.
His sisters are meant to be eternal. They could live forever, like him. They have the means to. And humans hurt them and hurt them, torture them until all they can do is scream, suck them dry until they're mere husks and then consume those husks to feed their neverending hunger. He might've felt differently, were they just being killed, but they're not, they're being hurt, they're being used like objects. So he is the blade that will cut off the parasites that are draining the life out of his sisters so that they can heal.