• 紹介しょうかい — introduction
• 仲介ちゅうかい — mediation; intermediation
• 介入かいにゅう — intervention
• 介護かいご — nursing care
• 魚介ぎょかい — marine products; seafood
• 厄介やっかい — troublesome; burden
• 意いに介かいさない — to take no notice of; pay no attention

介 originally shows a person standing between lines, suggesting “to mediate or be in between.” It also came to mean “shell” (protective cover) and extended to ideas of “help” or “intervention.”