Extra Gain is a system introduced in Disgaea: Hour of Darkness. In earlier games, characters created by using the Dark Assembly will become students/pupils of the character, who then becomes a master or mentor of the character. In Disgaea D2: A Brighter Darkness master/student relationships can be manually created by the player.
Depending on the game, the terms are slightly different. In Disgaea: Hour of Darkness, the creator of a character is named the "Mentor" and the created character the "Pupil," while in Disgaea 2: Cursed Memories and later games, the creator is named the "Master" and the created character the "Student".
Masters can learn spells from their students. In addition, the stats of masters will be slightly boosted as the students increase in un-equipped stats.
The Master/Student Relationship is one of the most powerful aspects of this game. It gives you the opportunity to have one character that knows many abilities from a wide variety of weapon classes.
When a character enters the dark assembly and chooses the "Create a Character" bill, that character becomes the master of the created character or the student.
This relationship is referred to in-game as Extra Gain.
When a master has students, the master will receive a bonus to their stats equal to 10% of the student with the highest statistics.
For example, Adell has three students.
For this scenario, Adell would receive a bonus to ATK of 10 from student 1, INT of 10 from student 2, and HIT of 10 from student 3. This bonus only applies for the base stats, i.e. without any equipment on the character.
When a student stands next to a master, the master can cast spells that the student knows. These spells include the elemental spells (Fire-Tera Fire, Ice-Tera Ice, Wind-Tera Wind, and Star-Tera Star), all of the healing spells, all of the debilitating spells (decreasing stats), and all of the boost spells (boosting stats). If the master casts this spell five times, the master learns it and can cast it as if he already knew it from his class.
Although it may be tempting to have every character learn every spell, pay mind to the Elemental Affinity. It would be more prudent (and save time) to teach the characters only the spells that they could cast with great effect. For some of the characters with no affinity, you could teach them all of them or merely pick one. Star and its higher level forms have no element associated, so they are not affected by affinity in any way.
Basic healing spells are also a must, but the boost spells are probably best left to the healers (unless you have a character who frequently plays support roles). The problem with having a lot of spells is that you never actually get them all to higher levels, making them rather pointless and short ranged. It is useful for every character to have a freeform ranged attack that can hit multiple enemies as well as their weapon skills or melee attack.
It is most efficient to have a "chain" of magic users starting with the mages (who have the highest INT and are the ones you want using magic). Frankly, healing power is so high that it doesn't matter who's doing it as long as they're holding staves. At the start of the chain you'd have the mages and at the end you'd have the clerics, so the healing spells "trickle up" the chain. Either three or four mages is optimal. Train them on turns where you have an easy 1-turn exit in the Item World.
Why have students?Edit
The bonus to stats is almost minimal at the higher levels, but the ability to learn spells is almost always a benefit. Even the knights and warriors can learn the spells and increase their capabilities on the battlefield.