Ada Clare and Esther Summerson are parallel characters—that is, characters who are very much alike in many ways. Both are young, pretty, self-effacing, good-natured, sensible, responsible, and delicate; both are orphaned, then eventually stationed in the same household; they have similar values and expectations of life; young men are attracted to both of them. They are also mutual confidants; they confide in each other, and partly because they do, they reveal aspects of their characters to us.
We learn far less about Ada (a clear example of a "minor" character); she remains in the background most of the time, whereas Esther is often "on center stage." Even so, Ada is both close to Esther and, through Richard, strongly involved in Jarndyce and Jarndyce; therefore, she is a more important minor character than say, Jobling (Weevle) or Watt Rouncewell.
In relation to Richard, both Ada and Esther are foil characters, that is, characters who in some important way contrast strongly with some other character and, through that contrast, make the other's character more distinct. Mature, realistic, prudent, and steadfast, Ada is all that Richard is not. In fact, Ada again, like Esther) expresses and represents normality and reality, the standards by which Dickens wants us to judge other characters. The strong sense of reality and normality with which Dickens endows both Ada and Esther gives these young women an important function in the story and prevents them from becoming mere figureheads—pretty but essentially useless objects of male desire and idealization.
Dickens emphasizes Ada's blonde, blue-eyed beauty. Might one make a plausible case for the idea that this emphasis, together with the fact that Ada remains, perhaps somewhat mysteriously and glamorously, in the background, gives to Ada, even more than to Esther, something of the power of the Archetypal or Eternal Feminine.
