Borachio is one of Don John’s followers. His name tells us well enough that he is a drunkard. He also has, however, a crafty mind.
He knows to bring any news that might serve as the basis for mischief to Don John. He is having a fling with Margaret, and has no qualms about using her as a prop in his plot for Don John without her knowledge. The fact that he is able to get as far as he does with the confirmed flirt Margaret suggests that he is a good dancer, as that appears to be a requirement for her favor. Unfortunately, Borachio does not know to hold his tongue when drunk, which leads to his and Conrad’s arrest and the discovery of the whole plot. The news of Hero’s death appears to bring out some remorse in him, and confesses in full to Don Pedro. He is also honest enough to spontaneously clear Margaret of all blame when it appears she’s about to be dragged into the mess.