Several parts combine into one character.

要 originated as a pictograph of a woman with both hands placed on her own waist — the waist being conceptually "the most central, essential part of the body" in ancient Chinese physiology. From this concrete image grew the abstract meanings: the central part → the most important / essential → what is needed → to demand. The original "waist" meaning was later given a dedicated character 腰 (yāo, "waist") by adding the body radical 月; this is a textbook case of one character splitting into two as a semantic drift created the need for disambiguation.

The semantic chain — waist → core → essential → necessary → required → demanded — runs cleanly through all CJK languages, making 要 one of the most heavily-loaded characters in everyday vocabulary.

Korean reading "yo." Saturates Korean abstract vocabulary: 重要 (jungyo, important — top-frequency adjective in formal Korean), 必要 (piryo, necessary), 要求 (yogu, demand / request — used legally, politically, and socially), 要約 (yoyak, summary / abstract), 主要 (juyo, main / principal), 要點 (yojeom, "essential point" = key point), 要素 (yoso, element / factor — used in mathematics, science, philosophy).

Mandarin yào, 4th tone (the verb 要求 takes yāo, 1st tone, on the first syllable due to historical phonetic rules). 要 (yào, to want / need / be going to), 重要 (zhòngyào, important), 主要 (zhǔyào, main), 需要 (xūyào, need / require), 要是 (yàoshi, "if / supposing" — colloquial conjunction). Critically, 要 functions as a modal/auxiliary verb in everyday Mandarin: 我要去 (wǒ yào qù, "I want to go / I am going to go"), 你要小心 (nǐ yào xiǎoxīn, "you must be careful"). This modal use makes 要 grammatically essential to sentence formation in Mandarin.

Japanese on-reading ヨウ (yō) — 重要 (jūyō, important), 必要 (hitsuyō, necessary), 要求 (yōkyū, demand), 要素 (yōso, element), 主要 (shuyō, main), 要点 (yōten, key point). Kun-readings: いる (iru, "to need / be necessary") — 要る (iru) — note this is a different verb from いる (居る, "to exist" — for animate beings). 要りますか (irimasu ka, "do you need it?") is everyday Japanese conversation. かなめ (kaname, "pivot / linchpin / cornerstone") — 要 (kaname) — used in formal Japanese: 組織の要 (soshiki no kaname, "the linchpin of the organization"). The kaname meaning preserves the original "central / essential" sense.

Memory aid: a woman with hands on her waist (the body's essential center) — the central, the necessary, the demanded.