Longaville is a nobleman in the King of Navarre's pack who eventually falls for the lovely Maria (of the Princess's pack). He wants to be Berowne. He's witty, too, as Maria recalls meeting him at a wedding. But he doesn't have Berowne's devil-may-care charm. He's a follower and is the first to sign the King's decree.
Boyet easily gets the better of Longaville in Act 2, withholding Maria's name and baiting his curiosity further. We have to conclude that his wit still needs some work. So does his writing. He is insecure about his love poem, which advances one argument—that because Maria is a goddess, not a woman, his oath does not apply to her.