Robert Greene was a rival playwright who famously attacked Shakespeare in 1592 as an "upstart crow, beautified in our feathers" in his pamphlet Greene's Groats-Worth of Wit, accusing him of plagiarism and being a mere actor-turned-writer. Greene's jealousy stemmed from Shakespeare's rapid success, challenging established university wits like himself. Greene's critique, though delivered posthumously, is the earliest surviving reference to Shakespeare as a writer, confirming his presence and success in London's theatre scene by the early 1590s. 

Key Aspects of Their Rivalry:

• The "Upstart Crow": Greene's insult called Shakespeare an "upstart" (newcomer) who was stealing the work ("feathers") of established playwrights, comparing him to a crow vainly adorned with borrowed plumage, says the WashU article.

• Accusations of Plagiarism: Greene suggested Shakespeare was taking material from other writers, including borrowing lines and imitating Christopher Marlowe's style, note the WashU article.

• Jealousy of an Actor: Greene, a Cambridge-educated playwright, resented Shakespeare, who was seen as an outsider and actor stealing scenes and success from university-trained writers, according to the Shakespeare Birthplace Trust.

• The Pamphlet's Significance: Greene's Groats-Worth of Wit, published in 1592, is crucial because it gives us concrete proof of Shakespeare's career at this early stage, despite Greene's vitriol, notes the Shakespeare Birthplace Trust article. 

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Robert Greene (1558-1592):

• A successful dramatist and writer known for his profligate lifestyle, says the TLS article.

• He died young in 1592, shortly after writing the famous attack on Shakespeare, as noted on Wikipedia