"Rehema ni mwaminifu"

Translation: Rehema is honest

[grayc_a]

so when describing people, an adjective would always start with mw-?

[Catriona28475]

It's not because the adjective describes a person; it's because it begins with a vowel. Then m- becomes mw-.

It's explained well in the notes & tips for the Adjectives lesson.