かすか

仄ほのかN1sense 1
ほのか also means faint or slight but often implies a gentle, pleasant subtlety (e.g., ほのかな香り), while かすか can describe anything barely perceptible regardless of pleasantness.
わずかN3sense 1
わずか emphasizes a small amount or degree, often meaning 'only' or 'mere', while かすか focuses on the indistinctness or faintness of perception itself.
おぼろげ sense 1
おぼろげ describes vague, hazy mental images or memories, while かすか is broader and covers faintness in any sense, not just recollection.
うっすらN1
微か is a na-adjective meaning 'faint, slight' used to describe barely perceptible sounds, light, or feelings, while うっすら is an adverb describing the manner in which something is faintly done or perceived.
ほんのり
微か describes something barely perceptible or faint in intensity, while ほんのり conveys a subtle but pleasant degree, often of warmth, color, or taste.

Opposites

はっきりN4sense 1
はっきり describes something clear and distinct, the opposite of かすか, which describes something faint and hard to perceive.

わずかN3
微か describes something faint or barely perceptible (a sound, light, memory), while 僅か describes a small quantity or narrow margin, not perceptibility.

かすかUsage79%
Usually written in kana; the kanji forms 微か and 幽か are rare and mainly literary.
微かすかUsage19%
Rare kanji form; 微 suggests faintness or subtlety.
幽かすかUsage2%
Rare kanji form; 幽 suggests dimness or seclusion.

The origin is uncertain. It may be related to classical Japanese かすか (kasuka) meaning 'faint' or 'dim', but the exact derivation is unclear.