必ず
かならず · kanarazu · JLPT N4 · adverb
certainly
必ず = kanara-zu, old 'not provisional': no maybe about it, so surely
One common etymology: kari (仮, provisional) + nara + zu (classical 'not') = not merely tentative, hence certain. The kanji 必 also means must, as in 必要 (hitsuyou, necessary), so 必ず is the 'without fail' adverb.
必 — heart 心 + slash 丿
Originally depicted a handle or marker driven through something, suggesting a fixed standard. The form was later reanalyzed as 心 (heart) with a stroke through it, reinforcing the sense of inner certainty. It came to mean 'must' or 'certainly'.