Leveling up is a common element that is used in nearly every RPG and even some other games. In The Last Story, it is used to make the player stronger and increase their stats. To level up the player requires experience, or XP, by killing enemies. You can kill enemies from dungeons, by using a summon circle, or by fighting in the Arena.
Summon circles are the red circular symbols found in rooms near a boss in a dungeon. You can activate the circle by standing on it and using Zael's Gathering. It should summon multiple enemies for you to fight. The more you fight in a circle, the less experience you will get. The XP you get depends on your level and the enemies' level. For example: If you are level 34 and the enemy is 28, you won't get nearly as much XP as if you were fighting a lv 40. So the higher level the enemy is, the more XP you get. Bosses also give you large amounts of XP.
The player's max level is 99. The monsters in NG+ can exceed that number, but if the player is 99 already, they cannot gain anymore experience. When you gain stats after leveling up, the numbers are completely random.
The playable characters are the only ones that can gain experience; the supporters (like Asthar and Therius) will remain the same until they leave your party. The characters in the party at the moment of the fight are the ones who will gain the experience, and sometimes some will raise faster than others since they appear in various chapters continuously. But near the end of the game, they all must be about the same level, unless the player alters it by fighting in the Arena.