counter for articles, individual
个 centers on counting individual items or referring to a single, separate entity. It is used as a counter for articles and to express the idea of an individual unit.
个 is a simplified variant of 箇, which originally used 竹 (bamboo) and 固 (solid) to indicate individual bamboo items. The modern form 个 is a radical simplification, and its exact historical development is uncertain.
The shape 个 looks like a single vertical stroke under a roof, like one item standing alone. Think of it as a counter marking one individual article.
For カ, imagine a car with a counter on the dashboard that counts each individual item: car -> カ, and the counter clicks for every article.