Who is Bottom in A Midsummer Night's Dream?

Nick Bottom is a weaver in Athens, and a member of a group of amateur performers comprised of fellow tradesmen. He is easily the loudest member of group, bombastic and enthusiastic -- so much so, in fact, that he often offers to play multiple roles within the same play! Not only that, he even pesters Peter Quince (the director/playwright) with suggestions of how the play should be performed. He's also very self-assured of his abilities, even though he frequently makes grammatical mistakes (e.g., using the word "odious" instead of "odours" in Act III, Scene i) and has questionable taste in poetry.

It might be possible to view him as a haughty, vain buffoon for this overconfidence, but he's also a very sympathetic character. His constant interfering with Peter Quince's play, for example, is not because he desires more glory onstage, but because he has a genuine passion for theatre. He lacks the prima donna's capacity for scheming: he acts like this only because he loves to act.

Although his fellow players may sometimes get exasperated by his actions, they also agree that he's a good friend a vital part of their small company:

FLUTE
No, he hath simply the best wit of any handicraft man in Athens.

QUINCE
Yea and the best person too; and he is a very paramour for a sweet voice.

(Act IV, Scene 2)

His most memorable role, though, is one in which he never knew he was acting: while rehearsing in the forest, Puck curses him to have the head of an ass, and Titania is cursed to fall in love with him. Bottom, being his confident and easy-going self, takes this all in stride, and is comically unaware of both his own condition and Titania's delusions. He is eventually restored to his former self, and although he remembers some things of his adventure with the Queen of the Fairies, he barely pauses before rushing off to rejoin his friends for their play.