一 (yī): The character for the number “one,” represented by a single horizontal stroke. It's the simplest Chinese character.
个 (gè): The most common measure word (or classifier) in Mandarin Chinese. Think of it as a general-purpose counter for people, items, and abstract concepts. It's the “unit” in “one unit of a person.”
人 (rén): The character for “person” or “human.” The pictograph resembles a person walking with two legs.
Together, 一 (one) + 个 (measure word) + 人 (person) forms the most basic way to say “a single person,” following the core Chinese grammar structure of “Number + Measure Word + Noun.”