Experience

Experience is a statistic in the Disgaea series. The more experience points that a character has, the higher their level. Characters can gain experience from killing enemies, objects, teammates (not advised, and gives out a reduced amount of experience), match bonuses, and by stealing it from enemies, objects or teammates with the Thief class.

Experience Formula for Disgaea 1-5
The formula used to determine the actual amount of experience it takes to reach a given level in games prior to Disgaea 6: Defiance of Destiny is based on a formula called "Enext", which appears to be irregular at the lower levels, especially at levels 11, 39, 65, and 99. The formula used seems to be an entirely different formula at different sets of levels, with levels beyond the first 100 using a much more flat progression than the first 98 levels. This formula can be seen here.

The experience formula is used for both determining how much experience it takes to reach the next level, as well as how much experience is given for defeating an enemy of that level.

Defeated enemies give you roughly 6 experience points plus 1/10th of the experience to reach the next level in their Enext formula as modified by their class's experience modifier. Higher-tier enemies give greater multipliers for defeating them compared to enemies of the same level.

Level 1-99
For the first 98 levels, the rate of increase in experience points needed to go to the next level (and the amount of experience an enemy of that level gives) rises irregularly but roughly parabolically. This pre-99 formula seems to be roughly .04 * X^3 + .8 * X^2 + 2 * X, where X is the current level. The total amount of experience to reach any given level is roughly 0.01 * X^4 + 0.29 * X^3 + 1.05 * X^2 + X (-30 at level 11 to 38, +60 at level 39 to 64, +90 from 65 to 98. Why these levels involve strange shifts is unknown, but seem to be manual adjustments in the formula by the game designers).

Level 99 is a special level - it costs slightly over twice as many experience points to hit level 100 as a normal level, and a level 99 enemy gives twice as much experience as the formula would normally allow (9,830 as opposed to level 98's 4,555 or level 100's 3705), roughly making them equivalent to level 323 enemies in terms of experience, but not mana or money. At least one grinding sweet spot per game takes advantage of this fact.

Level 100-9999
Beyond level 99, the formula transitions towards costing 305 more experience points per level than the level before it, forming a very regular and linear progression. This eventually comes to be nearly exactly 305 * X experience per level, where X is the level of the character, or 152.5 * ((X-1)^2 + (X-1)) for the total experience required to reach level X. The formula makes levels from 100 to roughly 650 have less experience per level than this formula indicates in order to make the overall formula wind up averaging the 152.5 (X^2 + X) without needing a constant to compensate for the oddness of the pre-level 100 experience formula.

Ultimately, the total amount of base experience required to reach level 9999 is roughly 15.25 billion exp times the character's experience multiplier. Most story characters, therefore, require roughly 76.25 billion exp to max out their levels, as they have an experience multiplier of 5.

Other Details
Because of the difference in the way that the experience formulas work at levels below 100 (exponential) and above 100 (linear), killing a level 20 enemy is worth roughly 5 times as much experience as a level 10 enemy, whereas a level 2000 enemy is worth only around twice as much experience as a level 1000 enemy.

On top of this base experience formula, every class has a set multiplier for both the experience it takes to gain a level and how much experience they give. Story characters, for example, have an experience multiplier of 5, and because reaching level 2 has an Enext value of 3, it takes 15 experience to gain their first level. Basic-tier generic classes like Skulls, Warriors, and Prinnies have an experience multiplier of 4, take 12 experience to reach level 2, and take 80% as much experience to gain the same number of levels as a story character or mid-tier character would.

Practically speaking, this means that simply fighting higher level enemies is more important than multipliers to experience or killing a larger number of enemies at levels below 100, whereas beyond level 100, there is little practical difference between killing 3 level 3000 enemies and 9 level 1000 enemies. Likewise, above around level 200, you can roughly estimate the experience point value of an enemy as 30.5 times its class multiplier times any other multipliers that factor in.

Boosting Experience Gains
Equipping a Statistician Specialist to any equipped weapons or armor will boost the experience you gain by 1% per level of the Statistician, up to a maximum of 300 (or 400% experience). These bonuses do not stack, so having two Statisticians at level 300 will still only yield 400% total experience.

Destroying allies and clones will only give you 1/4 of the normal experience (and mana and money) you would otherwise receive. (Disgaea 4's map creation also cuts you to 1/4 normal experience.) This is multiplied separately from the other modifiers, such as statisticians.

Other miscellaneous factors include:
 * Bosses (such as Mid-Boss in Disgaea: Hour of Darkness and Axel in Disgaea 2: Cursed Memories) grant a x1.5 experience bonus when they are defeated.
 * Item Generals in the Item World grant a x1.5 experience bonus when they are defeated.
 * Item Kings in the Item World will grant a x2 experience bonus when defeated.
 * Item Gods in the Item World grant a x2.5 experience bonus when defeated.
 * Item God 2s in the Item World grant a x3 experience bonus when defeated.
 * In Disgaea 3 and Disgaea 4, evilities like Violence can reduce experience gain, however, they are simply subtracted from other multipliers, and as such, a 50-point statistician can counteract its -50% experience gain.
 * In Disgaea 2: Cursed Memories and Dark Hero Days the statistician was removed from gameplay and was replaced by felonies which had many different effects, one of which also increased the amount of experience gained capping at 300%.