negativist (n.)

• brawler
• cynic
• defeatist
• defendant
• diehard
• disputant
• dissident
• filibuster
• filibusterer
• killjoy
• litigant
• malist
• naysayer
• nihilist
• objector
• obstructer
• obstructionist
• opposer
• pessimist
• plaintiff
• protester
• quarreler
• resister
• scrapper
• thwarter
• worrywart
• wrangler