I agree with mrs-campbell. Giles Corey is one of my favorite characters in the play, mostly because he's both a "character" and a man of character.
He does serve as comic relief, though that is not what we most remember him for after the final curtain is drawn or page is turned. He is hard of hearing, so he's easiy offended when he hears people incorrectly (which apparently happened between him and John Proctor, someone Giles has much respect for otherwise). He's also a very litigious kind of guy--when someone has wronged him, he surely wants his day in court. He's older, so he's not afraid to speak his mind...and he does. (My students always love the "fart" line, and it makes Giles seem kind of hip and cool.)
While those things are true, Giles is also a rather sweet and naive guy. His cursing (see "fart" above) is pretty tame for a grown man--even a Puritan man. He innocently assumes that one of his small court cases would be remembered and spoken of among the important judges in the area. Giles is uncomfortable with his wife always reading, but that's only because he isn't really a reader and is a little intimidated by it. Arthur Miller tells us Giles is a relatively new convert and doesn't know his memorized prayers, which is another endearing quality--who hasn't struggled to memorize something new? Once Corey realizes his questions have caused his younger wife Martha to be "somewhat mentioned" in court, then accused and sentenced as a witch, he is mortified. Again, we can all relate to saying or doing something which inadvertantly gets someone else in trouble. We can relate to him.
Finally, Giles dies a noble death. Unlike the others, who of course didn't deserve their fates, he got to choose to confess and live or remain silent and die--as he was dying. He had been a bit of a laughingstock throughout at least the later years of his life, but his last line leaves an indelible impression on an audience. "More weight." Simple but heroic words, but with them he is able to save his childrens' inheritance.
In all of this discussion about a play and a character, let's not lose sight of the fact that Giles Corey was a real person who really did die by being pressed with giant stones. Miller did his homework, and the basic facts about the man are true. When I take students to Boston for a senior trip, we always make a trip to Salem. After we wander around the old cemetery, I take them to the adjacent memorial to the 20 people who died in this awful time. Without fail, they stop the longest at the stone marker for Giles Corey. There's just something about this old curmudgeon with which an audience connects.