Undyne was created by Toby Fox for the video game Undertale, and was also designed by others involved, such as Temmie Chang.

[2] Other early concept designs created by Temmie were meant to look like an Axem Ranger's head from Super Mario RPG per Fox's vision.

She is first seen in a full suit of armor, though when her helmet is removed, she is shown to be a fish woman with red hair pulled back in a ponytail, blue skin, fangs, and only one eye.

[1] She was difficult for Fox to decide on a final design, both in terms of appearance and characterization, with the only consistent detail being that she would be the first boss to try to kill the protagonist.

The final concept only materialized for Fox once he began writing the monologue she spoke before her boss battle with the protagonist.

[1] When designing a "cute outfit" for Undyne, artist Gigi commented that it looked too much like jogging clothes.

[1] Her design was first revealed through a Steam digital trading card shortly before the release date illustrated by Gigi.

[4] When composing the theme in Undyne's first boss battle, Fox used the same trumpet sounds used by ZUN in the Touhou series.

[4] Her home was designed by artist Gigi, with the only notes given by Fox being that it needed to look funny when it catches on fire.

"[5] Both of her boss themes, "Spear of Justice" and "Battle Against a True Hero," use a combination of real and synthetic instruments.

[6] In Undertale, she appears as an antagonist to the protagonist, the human, attempting to kill them in order to extract their soul and use it to free the monsters from the Underground to fulfill Asgore's plans.

[citation needed] After the human learns more about Alphys' past and reencounters Asgore, Undyne and other monsters show up to stop the fight.

A Nendoroid figure of Undyne by Good Smile was released, featuring accessories such as her spear and fish-shaped mug, and multiple swappable faces.

[12] IGN staff praised her for her charm, which they explained comes from being able to see her from "multiple facets" depending on the player's play style.

This ranges from her being a "fairly intimidating warrior" if the player kills any monsters or, if they are doing the genocide route, the "hero of the game."

"[23] Polygon writer Allegra Frank also regarded her as one of the best video game women of 2015, saying that she was the best female character to choose from in Undertale.

"[7] She has also been praised as a favorite LGBTQ character by multiple critics, including Nintendo Life writer Kate Gray.

[25] Writer Max Delsohn discussed their gender journey and how they reacted negatively to certain media, including what they describe as "Alphys and Undyne's easy, lesbian love in a whimsical, faraway world" due to the complications they were undergoing.

[23] The boss battle in the genocide route Undyne was considered a favorite by Game Informer writer Daniel Tack.

They also cited how the ability to run away was removed during this fight, requiring players to follow Undyne's comment about needing to face her head on so long as the human's heart is locked in place.