Most enemies award the player EXP when defeated. The amount of EXP awarded depends on the enemy and also varies with the difference between your level and the enemy's level, with higher-leveled enemies giving more EXP. This is capped at maximum and minimum levels, to prevent exploits that allow killing very high-leveled enemies from becoming gamebreaking, and ensure that all enemies killed give at least 1 EXP. There are some situations where enemies do not give EXP, such as in the Arena, in the Ring of Spiders, and during the introduction obstacle course on the M.S. Solar.
Completing quests gives EXP in a similar manner to killing enemies. The EXP from quests does not drop off as steeply at high levels as EXP from enemies, and some quests will also reward a constant, level-independent sum of EXP, to ensure that the quest is worth completing even if it is missed while at an appropriate level.
Enemy EXP cannot be obtained while wearing Equipment with the Zero XP modifier. This modifier does not affect quest EXP, however. Having party members also affects the EXP obtained from enemies only, with one party member reducing it by 10% and two party members reducing it by 25%. The Trainer modifier directly increases or reduces EXP from all sources.
There is one event that adds EXP directly to the player, during the big battle at the end of the Rhombus Dungeon, to ensure the player is at level 3 when fighting the boss. This was originally intended to counter players who fought fewer enemies than intended through sequence breaking. However, the EXP formula was updated during early access, and the Rhombus Dungeon enemies were not changed correspondingly, so this event now gives a substantial amount of EXP to every player.
Every 1000 EXP collected rewards the player with a level. Levelling up rewards the player with 1 Circuit Point for each of the five Circuit trees, even inactive ones (although those CP cannot be seen or used yet). Levelling will also increase some or all of the base Stats.
The maximum possible level is 99, which is much higher than most players will reach. Even players trying for 100% Completion will likely only reach a level in the mid-60s for the main game, or the mid-70s for the A New Home DLC. To continue past this in the base game requires extensive grinding. Since nearly all enemies will give 1 EXP at this point, it is best to grind against very weak enemies like Hedgehags.
New Game Plus offers much easier methods to reach the level cap. The Get On My Level perk allows for higher-level enemies that continue giving significant EXP right up to the level cap, while the 4x Experience perk affects even enemies that would otherwise give 1 EXP. Alternately, the Sergey Hax perk can allow the player to grind against respawning Shredders in the Vermillion Dungeon, which match Lea's level and can quickly bring her to the level cap.
Lea's base stats increase with her level. This is generally an exponential increase, with small sinusoidal variations to create a pseudo-random effect. A table showing all base stats is given below.