('Rude Buster', as heard in Rude Buster)
'Rude Buster' is a motif representing...
...uh...
...something.
Look, what it represents isn't important.
What is important is that it's seemingly used as a sort of "A Section" for the 'Determination' leitmotif, as heard in Rude Buster and Lantern.
It also (sort of) appears at the beginning of the UNDERTALE PS4 Launch Trailer of all things, which also happens to contain the first appearance of the 'Don't Forget' motif.