Several parts combine into one character.

唇 is a phonetic-semantic compound: 辰 (chén, "5th of 12 zodiac branches / morning hours / shellfish-like" — providing the sound) + 口 (mouth). The composite reads as "the soft flesh spread around the mouth" = lips. The phonetic 辰 has a complex history (its original meaning was a kind of clam/shellfish), but here it serves only as the sound component, with the meaning carried by 口.

Korean reading "sun." Korean uses 唇 strictly in medical and linguistic contexts: 口脣 (guseun, lips — anatomical / medical), 脣音 (suneum, "lip-sound" = labial consonant — phonological term used in linguistics: ㅁ, ㅂ, ㅍ are 순음 in Korean), 朱脣 (juseun, "vermilion lips" — classical literary term for beautiful red lips, used in poetry). Modern Korean overwhelmingly uses native (ipsul) for everyday "lips."

Mandarin chún, 2nd tone. 唇 (chún), 嘴唇 (zuǐchún, "mouth-lips" = lips — the everyday compound, since 嘴 covers "mouth" colloquially), 唇膏 (chúngāo, "lip-paste" = lipstick), 唇齿 (chúnchǐ, "lips and teeth" — used in idiom 唇亡齿寒 chúnwáng chǐhán "when lips are gone, teeth feel cold" = mutual interdependence between two parties; classic Chinese geopolitical metaphor). 双唇音 (shuāngchúnyīn, bilabial consonant) — phonology vocabulary.

Japanese on-reading シン (shin) is almost unused. The kun-reading くちびる (kuchibiru, "lips") dominates: 唇 (kuchibiru), 上唇 (uwakuchibiru, upper lip), 下唇 (shitakuchibiru, lower lip — sometimes shitakuchibira). The Japanese cosmetic term 口紅 (kuchibeni, "mouth-red") for lipstick uses different kanji but the concept overlaps. Japanese idiom 唇を噛む (kuchibiru o kamu, "to bite the lip") = "to suppress strong emotion / hold back from crying or speaking out" — universal across CJK.

Memory aid: mouth (口) plus 辰 (sound) — the soft flesh framing the mouth = lips.