快い
こころよい · kokoroyoi · JLPT N1 · i-adj
pleasant
🌸 心 heart + 良い good = something good for the heart (kokoro + yoi)
Kokoro means heart and yoi means good, so the word names what sits well with the heart. The kanji 快 also means pleasant, which matches the meaning.
快 — heart radical 忄 + 夬 (phonetic)
快 is a semantic-phonetic compound. The heart radical 忄 carries the meaning, since the original sense referred to a feeling of pleasure or delight. The right side 夬 supplies the sound. The meaning later extended from 'pleased' to 'quick, fast', the dominant modern sense.