If a person reads "King Lear", one of the questions that arises from it, is what is the role, the function of the Fool in it? Just to provide comic relief for the audience? No, actually, he is Lear's conscience, given a body and mind of its own, rather like the talking cricket in "Pinocchio", and just like that cricket, he got a thoroughly thankless job, for Lear simply does not listen to him, even if it may appear differently from time to time like in act I, scene 5. Yet, despite the Fool's social immunity from repercussions (like a whipping), he is no more successful with the king than Kent and Cordelia - only his immunity, not "available" for Kent and Cordelia, creates an illusion that he succeeds when they failed. That is not so. Lear has his revelation in act 3; but, ironically, he does it on his own, with no assistance from the Fool, for the latter now starts to fade away (perhaps literally; there are some theories that he may have died due to some illness or hyperthermia caused by the storm or just from the way of life in Middle Ages), and that too creates the illusion of his success.
The Fool first appears in act I, scene 4. Incidentally, before that it is noted that the Fool has "pined away" since Cordelia has left for France (I. IV., 62-63). This note establishes the connection between Cordelia who has chosen honesty over flattery and suffers for it and the Fool who is also honest rather than flattering in his own way - and he proves that immediately, the moment he appears and hears that now-disguised Kent has been hired by Lear: "Let me hire him too; here's my coxcomb [fool's cap]" (I. IV., 82), effectively naming Kent a fool. When Lear asks him about it, the Fool switches his attention to Lear, saying "Sirrah, you were best take my coxcomb". (I. IV., 84) - i.e. he actually calls Lear a fool. Furthermore, soon afterwards, the Fool goes even further, claiming that Lear is not only a fool, but he has made himself a fool (Welland, p. 256), and only then Lear threatens him with a whipping, which is promptly forgotten. This scene sets the pattern of communication between Lear and his Fool: the Fool proclaims the obvious, the common sense, and Lear plainly does not understand it all; he does not hear what the Fool is really trying to say.
Incidentally, in an encyclopedia, a "fool" is defined as a person who could be either "unintelligent or thoughtless" - just exactly who Lear was in the first half of the play - or a jester. Put otherwise, Lear starts to act foolishly in scene 1 of the first act when he sends Cordelia away, he continues to act so when he does not listen to the Fool and others who have more common sense than he - and, consequently, Fate "demotes" him to the social level of an addle-brained Fool - as his Fool prophesies - "....thou wouldst make a good fool" (I. V. 31) - and is proven right: Lear is fooled (i.e. made fool) by his elder daughters, just as Fool told him, in act I, scene 2.
In scene 5 Lear and Fool talk some more as they depart Goneril's hospitality to go to Regan's; "...For though she's as like this as a crab's like an apple..." (I. V. 12), "She will taste as like this as a crab does to a crab." (I. V. 15), the Fool tells Lear. And Lear's reply can be summarized in "I did her wrong." (I. V. 20), and "....I would not be mad!" (insane, foolish) (I. V. 38), which ends act I. Thus, Lear still fails to realize what his Fool actually says.
In act II the Fool appears afresh in the middle of the act, when Kent is set in stocks, and once again the Fool shows the obvious to Lear and Kent: "Fathers that wear rags do make their children blind, but fathers that bear bags shall see their children kind. Fortune, that arrogant whore, ne'er turns the key to th' poor." (II. IV. 44-49). Again, the Fool acts like Lear's personal prophet: Regan and Cornwall treat Lear barely better than Goneril, who also arrives on scene for their final countdown: Lear is set out to fend for himself in a terrible storm, and with no one but his Fool for company, just as the Fool promised Lear: "I will tarry, the fool will stay/ And let the wise man fly" (Welland p. 258).
In the stormy scene II of act 3 Lear clearly begins to slide into madness, but simultaneously, his own demeanor begins to change: in the end of the scene (unlike its beginning, when Lear still sounds like his original self), he actually expresses concern for the Fool (III. II. 65-71). This is certainly a change from Lear in the previous scenes and acts, for there he, for all of his "more sinn'd against than sinning", (III. II. 57) never showed any concern for anybody - up till now, act III, when he actually begins to grow more open-hearted. When they (Lear, Kent, and Fool) come to a hovel, Lear tells the Fool: "In, boy; go first. You houseless poverty,--nay, get thee in." (III. IV. 26-27). This is very different from Lear of the first two acts. And in lines 31-36 of the same scene and act, Lear continues to scoff at himself at having been superfluous to Cordelia, Kent, and possibly other people. And thus, paradoxically, Lear, as he casts off his sanity, also casts away the restraints of the society upon him; he finally comprehends the truth, and in a certain way, he is on his road towards inner peace - put otherwise, now Lear no longer needs assistance to see the obvious for he sees it himself, just like the Fool saw it from the beginning. "...the Fool suggests that there is ambiguity in the words wisdom and folly...." (Welland, p. 258).
Consequently, while Lear grows more, well, Fool-ish compared to how he was in the beginning of the play, the actual Fool is fading away. He no longer tries to point the obvious to Lear because the latter can see it for himself, even if he has difficulties in expressing that, and, therefore, the Fool has to settle for stating the obvious instead, as he does in the lines 98-101 of the same scene (IV). Yet that is a poor replacement for his former speeches in the previous two acts, and consequently, the Fool continues to say less and less as the act goes on, eventually reducing himself to few one-liners, until he finally says "And I'll go to bed at noon," in scene 6, after which he vanishes from the play for good. Why? Besides the "natural" reasons given earlier, he probably was removed by Shakespeare himself, for Lear no longer needed him to show the obvious - he has learned to see the truth on his own. This situation brings Lear new problems, but the Fool is gone, and no longer cares.
So, this is the function of the Fool of "King Lear" - to be a brave soul and help Lear to see the truth - who failed, for Lear was a man who neither saw nor realized the truth of real life until it was too late.
Resources:
Shakespeare, William. The Tragedy of King Lear.Ed. Jay Halio, Cambridge: Cambridge University Press, 1992.
Welsford, Enid. The Fool: His Social and Literary History. Massachusetts: Gloucester, 1966.