Why did Harime Nui's eye not heal?

Question

Sometime before episode 12, we learn Harime Nui lost her left eye from Matoi Isshin slashing her with a half-scissor.

Around episode 20, Harime Nui is pierced through the chest by Ryuuko, also by a half-scissor, but that time, she did not experience pain and healed from this more serious wound instantly.

It is also revealed that Harime Nui was fused with Life Fibers from birth having been developed inside a Life Fiber cocoon.

Given these facts, why did

• Harime Nui experience pain when slashed by Isshin?

• Her eye not heal from the wound?

Answer 1

On the second day of an official full-season all-nighter screening on 30-31 May 2014 at Kadokawa Cinema Shinjuku, there was a talk/Q&A session. One of the questions was asking this issue:

When Nui's left eye was attacked by Dr. Matoi, even though the Scissor Blades was only one side, why didn't the wound recover?

which was answered by Kazuki Nakashima, the scenario writer of Kill la Kill,

Nui took her carelessness [of getting attacked] as a punishment, thus she left the wound alone.

Source: Privatter (Japanese), pixiv encyclopedia (Japanese)

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I'd conclude that the regeneration is not a passive trait that will be done automatically, but it needs the user's willingness.

Answer 2

Based on the Nui Harime wikia page, her eye was destroyed.

Nui was distracted by the sound of Ryūko calling for her father, allowing Isshin to destroy her left eye with the purple Scissor Blade.

It's also worth noting that the Scissor Blades were able to

prevent Nui's arms from regenerating when they were cut off.

The power of the Scissor Blades are known to be the one thing that can cut through Life Fibers, and this is likely what prevents the regeneration of Nui's body parts that were removed by them, as it destroys them.

As an aside, in many anime and shows, when a limb or body part is destroyed, it is not able to be regenerated. The best example I can think of is

In Claymore, when a claymore loses a limb, they can have it heal by reattaching the severed limb. However, when Clare's arm is destroyed, she has no arm until one is given to her by Irene.