Old but i didn't see this posted so here it is, copy-paste from gematsu, the thing that i find more interesting that each one of the 500 characters can be considered a quest and each one gives you a skill!
http://gematsu.com/2016/04/caligula-details-skill-acquisition-character-episodes-suzuna-thorn
Learn New Skills
There are over 500 students trapped in the world of Mobius. Each person has a unique name and profile, and although they have traumas and troubles in reality, they live a life where theyre forgotten in Mobius. Your ties to these people holds the key to returning home from Mobius.
By forming friendships with students, youll unlock the Causality Linker correlation graph within Mobius. At first glance, it may seem that student friendships are unlikely, but youll connect in surprising places, and something big might be stirring up in locations unnoticed by the Going Home Club.
Each student has their own unique problem. So if were only looking at the students number of problems, it can be said that there are more than 500 quests in the game.
By solving students problems, new skills will unlock. The correlation graph (pictured above) acts as the games skill tree.
Those skills can be set by the members of the Going Home Club. It takes the power born from the traumas and complexes of the students to further increase the power of the Going Home Club. It is power beyond what was already permitted to them.
■ Going Home Club Character Episodes
Caligula has character episodes that touch on the secrets of the Going Home Clubs members, which arent revealed if you only proceed with the main scenario. Only the player can perceive their pain until the very end. However, you will feel the danger within them, and will not be blamed if you turn back because you felt disgust.
Not only will you experience surprising developments and outlooks on the world, youll also enjoy various pleasant conversations with your fellow club members.
As well as their many graphic pains.
■ Characters
Suzuna Kagura (voiced by Minami Tanaka)
Suzunas Catharsis Effect takes the form of a long spear. Together with it manifests a big black hat big enough to hide her face, masking the girls gentle facial expression and eyes. In battle, Suzuna is an attacker and healer. She is a recovery and support specialist that handles regular attack duties like Guard Breaks and aerial pursuits easily. She brings a sense of stability to the party.
■ Ostinato Musicians
Thorn (voiced by Yuka Otsubo)
Character Sound Composer: cosMo@Bousou-P
Leader of the musicians. A beautiful girl with an abnormal image, she appears before the main characters with the timing describable as like only that of a phantom. As a music producer for μ, as well as the oldest creator, she gained popularity in the area of heavy, dark, and violent love songs. She has a relationship of mutual trust with μ, and μ obediently follows whatever Thorn says.
http://gematsu.com/2016/04/caligula-details-skill-acquisition-character-episodes-suzuna-thorn