At the start of Deltarune chapter 2 when you examine the computer in your bedroom the description talks about how the final boss of a game on there has rainbow wings. This is a reference to Asriel Dremurr's final form in Undertale.
According to Toby Fox, Deltarune (2018) is completely unrelated to the hit game Undertale (2015).
Deltarune is an anagram of Undertale.
Kris can fluently play the piano. In the Spamton Sweepstakes link maze, Noelle recounts how Kris would come over to her house and distract Noelle so they could play piano alone. There are multiple piano puzzles in Deltarune, most of which are controlled by the player. When the player doesn't control Kris, they play more fluently than what's been played in any other puzzle. The song they play is titled on Spotify as "Piano that might not be played that well", and it's an instrumental version of the credits theme "Don't Forget". If the player attempts to play the piano after Kris in Chapter 4 using Susie's sheet music, Susie will remark about how it sounded better the first time.
"Don't forget" "Beginning" "Friendship" and "Field of Hopes and Dreams" all share the similar song, but instrumented differently.