With the symbolic motif of fire and ice prevailing throughout Charlotte Bronte's narrative of Jane Eyre's struggle for independence in a Victorian society, her conflicts between passion and rationality are reflected in several characters, especially Mr. Edward Fairfax Rochester and St. John Rivers.  An observation in Chapter XXI of Jane's presents the dilemmas of passion and cold reason: 

Feeling without judgement is a washy draught indeed; but judgement untempered by feeling is too bitter and husky a morsel for human deglutition.

Representative of this "feeling without judgement" is Mr. Rochester, while St. John characterizes "judgement untempered by feeling."  Thus, these two male characters act as foils in Bronte's narrative: 

Their personalities

A Gothic hero, Rochester is unconventional, described as having "a dark face, with stern features and a heavy brow" and given to moods and irrational behavior and violent emotion.  In Chapter XIV, he tells Jane "...at this moment, I am paving hell with energy."  He is represented by the motif of fire; his mansion burns and he emerges from this fire cleansed of his excessive pride.

On the other hand, St. John is a handsome clergyman who is always in control of his emotions, if he experiences any.  As A Calvinist preacher, he is austere and inflexible.  Unlike Rochester, St. John is anything but explosive in his feelings.  For instance, in Chapter XXXV, Jane notes. 

Without one overt act of hostility, one upbraiding word he contrived to impress me momently with the conviction that I was put beyond the pale of his favor.

In the latter chapters, St. John insists that Jane should marry him because she has a duty to God, Rochester feels an affinity of spirit with Jane, understanding her passions.  For example, in Chapter XXIII as he and Jane converse in the garden, she explains her feelings, and Rochester understands 

it is my spirit that addresses your spirit; just as if both had passed through the grave, and we stood at God's feet, equal—as we are!”

“As we are!” repeated Mr. Rochester—“so,” he added, enclosing me in his arms, gathering me to his breast, pressing his lips on my lips: “so, Jane!”

He also perceives Jane as his equal while St. John maintains the more patriachal attitude characteristic of the Victorian Age. St. John is a man of cold reserve.  About him Jane complains,  "I could hardly comprehend his present frigidity."

Their pasts

Clearly, Mr. Rochester has a troubled past whereas St. John is without blemish, albeit sterile in his past.  At the same time, however, St. John is not comfortable in England while Mr. Rochester is an intrinsic part of his environment, even to the point of comparing himself to the trees on his property.

Their ideas about life and marriage

In Chapter XXIII, Jane mistakenly believes that Mr. Rochester plans to marry Miss Fairfax, but as Mr. Rochester explains the rumors of his marriage, he refers to the time that it was

...plainly intimated to you that it was my intention to put my old bachelor's neck into the sacred noose....

he reiterates his attitude toward marriage as a prison or death while St. John perceives marriage a more sacred duty.  His proposal of marriage is dispassionate, unlike that of Mr. Rochester. InChapter XXXIV, to Jane he eplains logically why she should be "a missionary's wife": I have made you my study for ten months." When Jane refuses him, he insists on the rightness of his decision.