The Gutsy Bat is Ness's ultimate weapon in EarthBound. It gives Ness a massive boost in his Guts Stat, and does not suffer the increased miss chance associated with the Casey Bat. It is rarely dropped by the Bionic Kraken, with a drop rate of only 1/128.
The Gutsy Bat adds 127 guts to Ness, resulting in a lot more critical hits and suffering from less mortal blows. The downside is there is very little time to enjoy it as it can only be found in The Cave of the Past.