Fleshing out a character

I don't mean the gory vision you may have in your mind! o_0

By fleshing out a character, I mean making them memorable through the lengthening and broadening of their history, giving them a distinct personality and changing them in ways that make them separate from everybody else.

Let's use my main character's friend as an example. Her name will be "Lyrie" for this. How could I make Lyrie portray her past in my story, and at the same time, her her be a memorable character with an in-depth history and suchlike?