They are one of the few characters to acknowledge the presence of the player, with whom they directly interact with.
The Voice is a mysterious character whose goals are not known. They talk to the player and offer to create a vessel for them.
The Voice's dialogue is entirely uppercase and the text displays slower than for other characters, suggesting that the Voice is a slow talker.
The Voice greets the player during the game's introduction, asking: "ARE YOU THERE? ARE WE CONNECTED?"
After that, the SOUL appears on the screen, to which the Voice asks the player to create a vessel. The vessel is shaped after a human looking like Kris, with options for its head, torso and legs, as well as its personality.
Once the vessel is done, the Voice asks for the vessel's, then the player's name. Once done, a different speaker says that the creation will be discarded since "no one can choose who they are in this world." This speaker is about to say the player's name when the scene cuts into Kris's bedroom, and Toriel shouts their name.
Upon getting a Game Over in Chapter 1, the Voice speaks to the player similarly to how they did in the intro, saying they appear to have reached an end and asking them if they want to try again. If the player chooses to continue, the Voice tells them the future is in their hands, after which the screen fades to white and goes back to normal gameplay. Alternatively, if the player chooses to give up, the Voice tells them that the world was covered in darkness, followed by the screen turning black and remaining that way while "Darkness Falls" plays in its entirety. On PC, the game then closes; on consoles, the game returns to the screen seen upon opening the game.
If the player closes and reopens the game, they are presented with a load menu. If Chapter 1 has not previously been completed, the menu narration is implied by the word choice and capitalization to be the Voice.
In Chapter 2, there is a proper Game Over screen and Ralsei or Susie speaks instead, urging the player to not give up. If neither in the party, nobody speaks and the player immediately gets the option to continue or give up. Choosing to continue has the screen fade to white and return to normal gameplay, and choosing to not plays out the exact same as in Chapter 1 (with the Voice returning as well), although on PC, the game returns to the chapter selection screen instead of closing.
Chapter 1 Pre-Release
Leading up to the announcement of Deltarune, the UnderTale and UndertaleJP Twitter accounts had their names and avatars blanked out, and began to tweet cryptic messages. These messages are written in-character, but for an unknown character. This character speaks in all capital letters in English, and in a mix of kanji and katakana in Japanese, which are the distinctive writing traits of the Voice.
In the first sequence of tweets, posted October 30, 2018, the unknown character notes that the reader has been looking for them, and that they likewise have been looking for the reader. They then say that they have something to show to the reader, but that it is not complete, so the reader should return in "" hours. They mention that they intend to use the reader's responses to "approach its realization," possibly a reference to the vessel creation process in the introduction.
In the second sequence of tweets, posted October 31, 2018, the unknown character thanks the reader for the patience, noting that they both have been waiting a "very long time," but are excited now that they and the reader are on the "verge of connection." They comment that they are looking forward to creating a new future with the reader, then link to the Deltarune website.
At the time, the Deltarune website included a message in same font and writing style as the voice, which advised the reader to read the warnings below, then "take it in [their] hands." The website included a download for a file named "SURVEY_PROGRAM," which was the installer for Deltarune Chapter 1.
When running the installer, instead of containing a license, the license text in the license agreement of the installer simply says "YOU ACCEPT EVERYTHING THAT WILL HAPPEN FROM NOW ON." This likewise is written in allcaps, indicating that this message was written by the voice.
Chapter 2 Pre-Release
After the announcement of Chapter 2, the UnderTale and UndertaleJP Twitter accounts once again tweeted in-character in the lead up to Chapter 2's release, in the unique style of the Voice. The voice noted that it had been a long time, and proposed that they could make a second connection.
- The name of the music track playing during the introduction is titled "ANOTHER HIM" in all capital letters, similar to how the Voice speaks. Several other songs in Deltarune's OST follow this pattern.
- In Undertale's files, Gaster's Theme is labeled "mus_st_him", possibly indicating a connection.
- The typer value used in the Introduction of Deltarune is "666" and the typer value "667" is used for the Game Over sequence. The typer value used for Entry 17 in Undertale is "666". This may imply a connection between Gaster and the Voice.
- Most in-game assets regarding the Voice are internally referred to using all capital letters.
- In the Japanese translation of Deltarune, the Voice's dialogues use a mix of kanji and katakana, an unusual writing style, which is not used by other characters.
- When the vessel is discarded, the voice telling the player this is implied to be a different character, due to using standard capitalization instead of allcaps.
- In Japanese, the second voice uses a mix of kanji and hiragana instead, different from the distinct writing style of the previous voice. Additionally, the second voice uses the second-person pronoun "omae," whereas the first voice uses "anata."