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Gameplay should come first in a video game. That does not mean that story should be just ignored. It isn't hard to be creative and give some kind of explanation. For example, just with what you gav...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/36327 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
Gameplay should come first in a video game. That does not mean that story should be just ignored. It isn't hard to be creative and give some kind of explanation. For example, just with what you gave us, I could say the NPC is ALWAYS invulnerable, but when he gets "tired" because he is being pounded on, to remain invulnerable, he becomes immaterial. That way he remains invulnerable (his primary ability) at the expense of being able to protect others. This could be conveyed visually by animations that show him getting tired, and then by his becoming somewhat translucent. After all, just because he remains invulnerable doesn't mean he must always be substantial.