Character Role Analysis
Gloucester's Family to Lear's Family
You don't just have one-on-one foil action in Lear, you have a family-on-family foil feud. Say that five times fast.
King Lear's complicated system of foils, in which Gloucester's family reflects Lear's family, is famous. As poet William Butler Yeats once wrote, "Lear's shadow is in Gloucester, who also has ungrateful children, and the mind goes on imagining other shadows, shadow after shadow until it has pictured the whole world."
Or—in simpler terms—both families feature a good-but-misguided father figure deceived by his bad children and who mistreats his good child. Arguably, this makes the play universal; it's not that one family is messed up, but that all families have problems.
Earl of Gloucester to King Lear
Within the families-as-foils dynamic, you do find one individual foiltastic relationship: Gloucester and Lear.
These guys are both suffering from pain inflicted by members of their families. But the patriarchs are similar in other ways, as well: Gloucester is blinded physically in the course of the play, and Lear is mentally blinded when he goes mad. Neither fully appreciates the good that those loyal to them have done (Lear doesn't give Kent his verbal kudos, and Gloucester dies before he can really do anything for Edgar). Lear dies when Cordelia is lost to him, whereas Gloucester dies when Edgar is (again, metaphorically) brought back to life before him.
Kent to Oswald
Kent and Oswald are both servants, but Kent is way more loyal than Oswald is. That's not saying much, though.
Even after Lear banishes Kent, the man remains true to Lear by disguising himself as "Caius," in order to serve the king. At the end of the play, when Kent is offered the job of co-ruling the country with Edgar, he refuses, implying that he will commit suicide so he can be with his "master," King Lear.
Some literary critics see Kent as being an emblem of an old school style of service, whereas his counterpart, Oswald, seems to embody a newer model of service—that is, Oswald is motivated by self-interest rather than loyalty and puts his own needs and desires ahead of his master's. When Kent and Oswald get into a slapping contest in Act 2, Scene 2, Shakespeare seems to be pitting these two styles of service against each other.