Alright.

So, I’d like to start off with the disclaimer that Undyne is of course not aesthetically an anime girl, and this interpretation of her character is entirely based on her personality, actions and occurrences.

Undyne is a big fan of shounen anime, as judged by several different instances of dialogue where she mentions people toting giant swords, and this is very likely a deliberate reference to her own character – Undyne is a shounen protagonist.
In the Neutral and Pacifist routes, Undyne, on purpose, waits for Frisk to arrive balancing on the peak of a mountain, just so that she could dramatically pose and *look* like the hero she is. Not only that, but her speech to the human before the fight seems to have been intended to be the “motivations & reasons talk the hero gives to the bad guy” but, since this is Undertale, she subverts all expectations by having a fit of rage. And then gives her speech anyways. Not only that, but if Undyne is killed, her death is extremely dramatic and honestly pretty heart-breaking, like most anime character deaths.
But everything so far is just an interesting parallel. Now I give you the **proof.**

Undyne, in the Genocide route, gets in the way of the human to protect Monster Kid. The noble act cost dearly, as her HP is instantly reduced to 0.
However, she does not immediately die. At the cusp of death, her body and soul already beginning to disintegrate, she remembers what is at stake. What happens if she loses. Her friends, her entire kind, and even her *sworn enemies* would be wiped out. And right there, on the crevasse bridge, she feels everyone’s souls empowering her. She may be on the brink of death, but she’s the heroine who NEVER gives up. And she is **determined.** She, against all odds, rises up against the evil force threatening to destroy everything, and will not give up until either one is killed.