In Mary Shelley's Frankenstein, Victor characterizes both Elizabeth and Clerval by contrasting them with himself.
In chapter two, Victor characterizes Elizabeth by setting her against himself.
Elizabeth was of a calmer and more concentrated disposition; but, with all my ardour, I was capable of a more intense application and was more deeply smitten with the thirst for knowledge.
This shows Elizabeth to be far more concerned with the nature of the world over the science of the world. This idea is supported by the following.
While my companion contemplated with a serious and satisfied spirit the magnificent appearances of things, I delighted in investigating their causes.
Therefore, Victor contrasts Elizabeth's love for the natural with his own love for science. This, therefore, sets Elizabeth as a foil to Victor given her love of the natural exists as a polar opposite of science.
Henry Clerval is much like Elizabeth. This again sets Clerval in contrast to Victor.
He was a boy of singular talent and fancy. He loved enterprise, hardship, and even danger, for its own sake. He was deeply read in books of chivalry and romance. He composed heroic songs, and began to write many a tale of enchantment and knightly adventure.
Here, Victor shows Clerval's love of adventure and fantasy. Here though, one can see a link between Clerval and Victor--they both love creation. Contrastingly, Clerval loves to create stories of fantasy while Victor's desire to create lies in actual creation of a being.
Essentially, Victor sets to describe both Elizabeth and Clerval as being opposites of himself. While both Elizabeth and Clerval love the romantic side of life, Victor finds more pleasure in the scientific aspects of life. Therefore, Elizabeth and Clerval seem to love what could be, Victor loves what he can make happen.