伴

Left person radical 亻 — the side-form of 人. Anchors 伴 in the human-relation family alongside 你, 他, 们, 仁. Carries the meaning core: a companion is by definition another person, so the radical does not just label the character, it embodies the concept.

Right component supplies the sound: 半 bàn → bàn (exact match). Beautifully transparent semantically too: a companion is your 'other half,' so 亻 + 半 reads as 'the person who is your half.' Among learners this is one of the cleanest mnemonics in the whole language. Same phonetic in 拌 bàn (mix), 绊 bàn (trip up), 胖 pàng (plump).