How do I get Jevil Deltarune?
Keeping this in view, how do you get the secret boss in Deltarune?
Go back downstairs and talk to the jail bars. When given the option to use the Prison Key, use it. A door on the bars will open, granting you access to the secret boss. There's a save point right there, which you'll probably want to use, because this boss is by far the hardest enemy in Deltarune.
Considering this, where is the shopkeeper in Deltarune?
Seam (pronounced Shawm) is the first shopkeeper encountered in the game, found in the Field.
The growing and shrinking sound effects of K. Round are a reference to the Super Mushroom sound effects from the Mario series. If the damage of the party is modified by external means, it is possible to "kill" K. Round, knocking it back much like C.