Consider how Black Widow first was in the second Iron Man, I believe.
Characters not yet fully developed relative to the protagonist, i.e. supporting characters that need less detailed writing, can be mysterious enough to capture initial curiosity, and eventually garner sufficient consumer intrigue for their own feature film to be produced.
As the attention directed toward them goes, the more profitable a film/series focused on them promises to be, the more character development they warrant - until they encounter newer characters with fresher appeals.