Borachio is one of the henchmen of Don John, the prince's evil half-brother. Interestingly enough, the name "Borachio" comes from the Spanish word for "drunkard," so we get the idea that Shakespeare intended Borachio to be a partier - probably a bit more interested in drinking and wenching than in very much else.
Borachio was willing to help Don John thwart the wedding plans between Hero and Claudio by setting up a scenario in which Hero appears to be cheating on Claudio with Borachio himself (it is actually Hero's maid, Margaret, but Don John makes sure that Claudio and the prince believe it to be Hero). Don John paid him 1,000 ducats to make this plan successful, and it is afterward, when he is bragging to their friend, Conrade, about his success in this matter, that both Conrade and Borachio are arrested for doing this awful thing to Hero and Claudio.
One might think that Borachio is just a big, selfish jerk who has no conscience. Although it is true that he is thoughtless and doesn't initially consider the ramifications of his actions, he did show sincere regret and remorse when told that Hero died as a result of the public disgrace. He confessed his wrongdoing to Leonato, Hero's father, and apologized sincerely, desiring that Leonato give him whatever punishment he saw fit.
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