The Fool is Lear’s own stand-up comedian, but he's the only guy that Lear allows to criticize him. But in Shakespeare’s plays, he seems to be very intelligent and also a good person.
Fool assumes to be Lear’s protector. The Fool is the King's advocate, loyal and honest, but he is also able to point out the king's faults, as no one else can. The Fool shares his master's fate, and this reinforces the impression that the Fool's purpose is to protect Lear until Cordelia can arrive to help her father. Edgar acts like the child that Lear needs, but without the Fool to civilize him, the audience would not feel as linked to Lear
He only appears in the fourth scene, and exits never to return in Act 3, Sc. vi some say that the parts of Cordelia and the Fool were played by the same actor The only possible reference to the Fool after that is in the final scene, when King Lear says "And my poor fool is hanged" (5.3.17). The Fool fades from the play when he is no longer needed. Sometime on the road to Dover, never recovering from the loss of Cordelia, the Fool hangs himself in grief.
The Fool serves a definite purpose, he is the King's closest friend and he is very critic. His position and personal qualities enable him to speak frankly to all those he meets, with a sort of 'humorous immunity' from the punishments that Kent and Cordelia have to endure. The Fool is able to tell the King uncomfortable home truths partly because of his ability to run rings around Lear intellectually. The Fool is a 'jumpy' character in many ways; at times he has incredible confidence in himself and his message and is able to express himself cleverly and with humour, but when he becomes frightened by Lear's spectacular reaction to his daughters' double rejection, his jokes lose their impact, and he becomes the King's nervous companion.
Some important quotes
1. Fool: Truth's a dog must to kennel; he must be whipped out, when the Lady Brach may stand by the