个 gè traces back to a single stalk of bamboo. The traditional form 個 is a phonetic-semantic compound: 亻 (rén, person) supplies the human sense of a single individual, and 固 (gù) supplies the sound. But the simplified 个 reaches further back, to an old graph that pictured one half of 竹 (zhú, bamboo). 竹 is two bamboo leaves side by side; split it down the middle and you are left with the shape of 个, a single drooping leaf. Counting stalks one by one is one of the oldest uses of a tally word, which is exactly the job 个 still does.

From "one stalk, one piece" the character settles into two everyday roles:

1. The universal measure word. Chinese counts nouns with a classifier between the number and the noun, and 个 is the default one, reached for whenever no more specific classifier fits: 一个人 yí ge rén (one person), 一个问题 yí ge wèntí (one question), 三个苹果 sān ge píngguǒ (three apples). In speech it is usually neutral-toned (ge) and is by far the most common classifier in the language.
2. Individual, single. The same "one piece" sense, applied to a single member of a group: 个人 gèrén (the individual; personal), 个性 gèxìng (personality, what is particular to one person), 个别 gèbié (individual; a few).

The thread is singleness. Whether 个 is tallying apples or naming the lone self set apart from the crowd, it picks out one unit from many. To count a thing is to treat it as a discrete piece, and that same act of picking out one is what makes 个 the root of "the individual."

Chinese does not say "three apples" directly. It says "three [classifier] apples," 三个苹果, with a measure word naming the kind of thing being counted. Many nouns take a specific classifier: 本 běn for bound books, 张 zhāng for flat sheets, 只 zhī for many animals, 条 tiáo for long thin things. 个 gè is the general-purpose default, the one you reach for when no special classifier comes to mind, and the one that covers people, abstract ideas, and most ordinary objects.

Because it fits almost anywhere, 个 is the first classifier a learner meets and the safest fallback in conversation: 一个人 (a person), 一个想法 (an idea), 一个星期 (a week), 那个东西 (that thing). It is also the workhorse behind 这个 zhège (this one) and 那个 nàge (that one). When 个 is doing this counting job, it usually drops to a neutral tone (ge) in fluent speech, even though its citation tone is the falling fourth (gè).

See one stalk of bamboo standing alone, a single leaf drooping from it. That lone stalk is 个: one piece, picked out from the grove. To count is to pull out one stalk at a time, which is why 个 is the measure word you reach for when nothing more specific fits: 一个人, 一个问题, 一个想法.

Now let the stalk become a person. The same act of singling out one unit gives you 个人 (the individual), 个性 (what makes that one person distinct), 个子 (the build of that single frame). Wherever 个 appears, look for the one piece set apart from the many: counted as a unit, or standing on its own as an individual.