This one is more of an oddity than anything you really need to know.  However, it’s oddities like this that are not generally taught, and you kind of have to discover them for yourself.  That’s what this site is for!

There are situations where a katakana ke (ケ) or a small version of that character is used for a kind of shorthand for kanji.  It appears that this is used in several situations, either as a counter  (for months or small things, for example), or as a part of a placename.  In most cases, it’s a substitute for a specific kanji, and has a specific pronunciation that is not “ke”.

One example of use is in a placename is 青木ヶ原, or aokigahara.

See this wikipedia page for a more exhaustive treatise on what this is for.