[1] While identical in shape to a small ケ, ヶ is actually an abbreviation for the kanji 箇, specifically by writing half of the bamboo radical 竹 (⺮).
箇, alternatively written as 個 (or 个), is a common Japanese counter word.
For example, 個人 kojin, "individual" will not be written as ×ヶ人 (except possibly as very informal ryakuji; contrast with 〆, as an abbreviation for 締).
One relatively common word using ヶ is 雁ヶ音 karigane (kari-ga-ne – goose-'s-sound, the cry of the wild goose).
It may also be a counter, where it will generally be pronounced ka, as in 三ヶ日 (Mikkabi, place name, "three days").