The Statistician is an innocent found in every Disgaea game except for Disgaea 2: Cursed Memories as they were replaced by Felonies. They increase experience depending the value of the Statistician (i.e. a 30 Statistician would increase EXP by 30%). They are capped at 300 each and it is possible to create multiple stacks of Statisticians for extremely fast leveling (for example, 32 stacks of 300 Statisticians would yield 9600% EXP and makes it possible to reach Level 9999 in two fights at Land of Carnage House of Ordeals 4). In Disgaea 3 and 4, they are found through Reverse Pirating.
Statisticians in Disgaea: Hour of Darkness are capped at 300% the EXP gain total, combined across all Statisticians in all equipped items. To create multiple stacks in later games, use the Puppy Paw Stick to duplicate an Item with a Statistician and fuse them until there is a 300 one before making multiple stacks.
The effective cap for Statisticians in Disgaea: Hour of Darkness is 300% (though the max for a single Statistician is 600), in Disgaea 3: Absence of Justice, it is 9600%, in Disgaea 4: A Promise Unforgotten and Disgaea D2: A Brighter Darkness, it is 4900%.