Several parts combine into one character.

The etymology of 必 is debated, with several theories competing. The leading interpretation: 必 originally depicted a weapon with a firmly gripped handle, conveying "to grip firmly / to be certain of." Other scholars read it as a deliberate dot added inside or beside an enclosure to mark "definitely so." 必 is filed under the 心 (heart) radical in dictionaries, which gives the impression that the meaning involves "the heart's certainty," but this radical placement is largely a graphical accident — the bottom portion resembles 心 but doesn't derive from it. Whatever its origin, the abstract sense became firmly established: "necessarily, certainly, must."

Korean reading "pil." 必須 (pilsu, essential / required — used in pilsu-gwamok "required course"), 必要 (piryo, necessary — paired with the previous character 要), 必勝 (pilseung, "must win" — battle cry on banners and team mottos: pilseung-ui uiji "the will to win"), 必死 (pilsa, "must die" = desperate / life-or-death), 必然 (pilyeon, inevitability — philosophy term: pilyeonseong "necessity"), and the lovely 何必 (hapil, "why must / why of all things" — used to express frustrated wonder: hapil oneul "of all days, today"). Hapil is one of the most charming Korean discourse particles.

Mandarin bì, 4th tone. 必 (bì), 必须 (bìxū, "must / necessarily" — extremely common in formal and instructional Chinese: 必须按时 bìxū ànshí, "must be on time"), 必要 (bìyào, necessary), 必然 (bìrán, inevitable / certainly), 未必 (wèibì, "not necessarily" — useful negation), 务必 (wùbì, "by all means / be sure to"). 必 is essential in Mandarin formal vocabulary.

Japanese on-reading ヒツ (hitsu) — 必要 (hitsuyō, necessary — pairs with 不必要 fu-hitsuyō "unnecessary"), 必死 (hisshi, "must-die" = desperate / for dear life — used to describe extreme effort), 必須 (hissu, essential), 必勝 (hisshō, "must-win" — Japanese sports cheers and martial arts mottos). Kun-reading かならず (kanarazu, "certainly / without fail / always") — 必ず (kanarazu) — top-frequency adverb in Japanese: 必ずやる (kanarazu yaru, "I will definitely do it"), 必ず来る (kanarazu kuru, "they will surely come"). The negative form 必ずしも~ない (kanarazu shimo ~ nai) means "not necessarily."

Memory aid: a firmly gripped handle / weapon — what is held tight is what one is certain of. The character looks like 心 (heart) but isn't etymologically related; the heart radical is graphical coincidence.