First up as a one who has been promoted as providing evidence of William Shakspere's authorship is Robert Greene.
Robert Greene (1558-92) is one contemporary of Shake-Speare often brought forward as providing evidence of William Shakspere's authorship. Greene was a poet, playwright and pamphleteer. In 1592 shortly before his death, he published a pamphlet Groatsworth of Wit, and annexed to it a letter to three unnamed author acquaintances who can be identified as Marlowe and (probably) Thomas Nashe and George Peele. The letter attacks actors for battening on dramatists and includes the following passage:
"Base-minded men all three of you, if by my misery you be not warned: for unto none of you (like me) sought those burrs to cleave: those puppets (I mean) that spake from our mouths, those antics garnished in our colours. Is it not strange that I, to whom they have all been beholding, shall (were ye in that case as I am now) be both at once of them forsaken? Yes, trust them not: for there is an upstart crow, beautified with our feathers, that with his tiger's heart wrapped in a player's hide supposes he is as well able to bombast out a blank verse as the best of you; and, being an absolute Johannes Factotum, is in his own conceit the only Shake-scene in a country".
The upstart crow was obviously Shakespeare; Shake-scene is a play on his name, and the words italicised are a parody of 3 Henry VI, 1.4.137: "O, Tiger's heart wrapped in a woman's hide". And Greene's crow was both actor and playwright. So Greene identified Shakspere with Shake-Speare. But plainly Greene was not a confidant of Shakspere of Stratford, but an enemy; and would therefore be unlikely to know Shakspere's secret, if there was one.