Learn Arabic (Modern Standard): صديقي خفيف الظل. - My friend is good company lit. light of shadow.

In Arabic, a person's shadow (الظل) is a metaphor for their presence or "vibe." If their shadow is خفيف (light), it means they are pleasant, easygoing, and have a good sense of humor. You will also often hear the exact opposite: ثقيل الظل (heavy of shadow), which describes someone who is annoying, boring, or a drag to be around.

Structurally, why does الظل have the definite article الـ while خفيف does not?

This phrase uses a structure often called a "false idafa" (or adjective idafa). Even though خفيف is an adjective, it acts as the first part of an ownership phrase (idafa), so it cannot take الـ. The second noun, الظل, specifies what is "light" and takes the definite article. Together, they form a single descriptive phrase acting as the predicate of the sentence.

If I want to say my female friend is good company, do I make the whole phrase feminine?

Not the whole phrase, only the subject and the adjective part! صديقي becomes صديقتي (my female friend), and خفيف becomes خفيفة. However, الظل stays exactly the same because it literally means "the shadow" (a fixed masculine noun). The correct sentence for a female friend is: صديقتي خفيفة الظل.