Heathcliff is an orphan, dark-skinned, passionate, violent, emotional, raw, extreme in rage and love.
Edgar is privileged, pale, bookish, cowardly, meek, tender, womanly.
Regarding Edgar, Heathcliff says:
"I wish you joy of the milk-blooded coward, Cathy!" said her friend. "I compliment you on your taste: and that is the slavering, shivering thing you preferred to me! I would not strike him with my fist, but I'd kick him with my foot, Is he weeping, or is he going to faint for fear?"
Look how Nelly describes Edgar:
"And Edgar standing solemnly by to see it over; then offering prayers of thanks to God for restoring peace to his house, and going back to his books! What in the name of all that feels, has he to do with books, when I am dying?"
Look how Edgar nurses Cathy: "No mother could have nursed an only child more devotedly than Edgar tended her."
And Nelly re: Edgar: "Mr Edgar's coldness depressed me exceedingly."
And: "Mr Edgar was taken with a nervous trembling, and his countenance grew deadly pale."
Contrast this with Heathcliff's countenance:
"He had grown a tall, athletic, well-formed man; beside whom, my master seemed quite slender and youth-like. His upright carriage suggested the idea of his having been in the army. His countenance was much older in expression, and decision of feature than Mr Linton's; it looked intelligent, and retained no marks of former degradation. A half-civilized ferocity lurked yet in the depressed brows, and eyes full of black fire, but it was subdued; and his manner was even dignified, quite divested of roughness though too stern for grace."