> How does this game relate to UNDERTALE? Is it a sequel?
Please don't worry too much about that.
Essentially, this is not the world of UNDERTALE.
If you completed UNDERTALE, the ending and world are as you left it.
If everyone was happy there, the people in the UNDERTALE world will still be happy.
So, please don't worry about those characters, and that world. It will remain untouched.
To rephrase that, DELTARUNE's world is a different one.
With different characters, that have lived different lives.