Is 'worse' the only comparative that has neither -er nor more?

Much/many, more, most and little/few, less/fewer, least/fewest are the positive, comparative, and superlative quantifiers in English. They come in two scales, with affirmative and negative increments.

Note that the negative scale distinguishes count (few, fewer, fewest) from mass (little, less, least) consistently, while the affirmative scale does so only in the positive degree (much, many); more and most don't distinguish mass from count - fewer people, less cheese; more people, more cheese.

It's not unique. "Less" is a valid comparative of "little."