I don’t think there is anyway to describe this prompt other than terrifying. At least if you met someone in real life who did what the girl in this prompt did and then said what this girl said. You probably won’t because that would be a very bad, potentially fatal wound.
Now, nothing says she wasn’t fatally wounded. Yes, she is smiling and claiming it doesn’t hurt, but it’s possible that she’s trying not to give her killer the satisfaction of seeing her cry.
On the other hand, maybe she really isn’t hurt by the knife to the chest. Maybe she really is as unfazed as she seems. Obviously if that was the case, she’s either got a really good shield or thick clothes of some kind or this is not something that can happen in reality.
Personally I am inclined to believe that is the case that this is some sort of sci-fi or fantasy story and she is immortal, can’t feel pain, has no blood, or some such thing. I not even sure what to say to narrow it down, but I think she really isn’t hurt and that smile should send a chill down your spine. Maybe she’s not human. Maybe she has a body the size of a cat and walks around in a robotic suit that looks like a human similar to that one alien in Men in Black. Maybe her organs are in a different place and it does hurt, but not that much or at least it’s not fatal and she’s ready for vengeance against the person who tried to kill her.
Beyond the creep factor, I see the potential for the character in this prompt to be either a villain or a protagonist or at least as someone who is a major character on one side or the other. She doesn’t seem the type to be on the sidelines, though it might be interesting if you had a story that worked with her as a minor character.
This situation and the corresponding line would, to my mind, be most useful as the first time we learn about this power, though if she’s the protagonist or someone close to the protagonist, there should be something that hints at this prior to that. Maybe she never got hurt as a kid. Maybe fell down something that shouldn’t have left her unharmed and somehow was unharmed. If she does know about this, as she seems to, maybe she volunteers for the most dangerous missions in order to protect those who are not as sturdy as she is.
Or maybe she’s normal, average. The people up against her are from somewhere else and they don’t know this, but for the area they are in or where she’s from, this ability is just normal and not at all what they are expecting.
I love all the potential this prompt has. There is so much you can do with it and two people taking the same prompt are likely to go completely different ways. One hundred people taking the same prompt are still unlikely to write the character the same way and definitely will have different stories. What story would you tell with a character like this?