Kent is introduced at the very beginning of this play as a character whose defining characteristic is honesty and loyalty. This is something that so defines him that he will even stand up against his monarch and face punishment for doing so. Kent is a character who is unable to sit by and watch his king being manipulated by his daughters and then also watch Lear make a mistake that he knows he will regret in the future. This is why he speaks up and tries to reason with Lear and tries to encourage him to take back his hasty judgement against Cordelia. Note what he says about his role and how, in defying Lear, he is merely exhibiting once more his loyalty:
My life I never held but as a pawn
To wage against thy enemies, nor fear to lose it,
Thy safety being motive.
In Kent's eyes, his entire life has been lived to serve Lear as a "pawn," and this continues, even if the enemy that he fights for Lear is Lear himself. Even though he faces death, he will gladly give his life for the safety of his liege. This is why Kent is the one character who stands up for Cordelia. Both he and Cordelia act as foils for Regan and Goneril in this scene, as they speak the truth and are punished for it, whereas the two evil sisters speak lies and are rewarded for it.