• 快感かいかん — pleasant feeling
• 快適かいてき — comfortable
• 快活かいかつ — cheerful, lively
• 愉快ゆかい — delightful, fun
• 明快めいかい — clear and concise

快 is a 形声 (keisei) character. The left radical 忄 represents the heart or emotion, while the right part 夬 (pronounced クワイ in on-yomi) gives the sound. Together, they express feelings that uplift the heart — like pleasure or comfort.

このホテルは とても快適かいてきです。
This hotel is very comfortable.

彼かれは 快活かいかつな 性格せいかくです。
He has a cheerful personality.

快 is usually read as カイ in compound words like 快適 or 快感. The kun-yomi こころよい is more poetic or literary, often used in descriptive writing or classic expressions.