As charisma is always in low demand by player characters (cause which GM wants to spoil every adventure by continued "they wont talk to you" attitude..), sanity controles the mental boundary that characters have to follow.
Sanity represents a character's mental well-being - or, more accurately, his capacity for mental well-being. Sanity represents the upper limit of a character's health. A character can behave below his Sanity level but not above. Each Sanity score represents a certain stage in mental development. How a character acts at each stage is largely dependent on his own basic anxieties or fears, but certain distinctive behavior patterns typify each stage.
|11+||LIBERATED. This character is free from basic fears and anxieties and has developed himself to his fullest potential.|
|10||SELF-AWARE. This character's basic fear feeds an inner desire, which he satisfies by developing a specific skill that gives him confidence and keeps him well-balanced.|
|9||CONSTRUCTIVE. This character, acting to overcome secondary fears, defines himself through a social role and holds close to a characteristic identity - quirky, perhaps, but still basically healthy.|
|8||IMBALANCED. This character has begun to succumb to a temptation that contradicts his best interest, taking a personality trait beyond its usefulness, but he still relates fairly well to the world at large.|
|7||DEFENSIVE. This character has reacted to his fears by trying to control himself or his environment.|
|6||OFFENSIVE. This character has overcompensated for his fears, resulting in selfish, extreme and usually objectionable behavior.|
|5||DISTURBED. This character has succumbed to his fears and has lapsed into defense mechanisms that create serious interpersonal conflict.|
|4||DELUDED. This character has opted to reconstruct reality rather than face up to it, often resulting in compulsive behavior. He can be considered fully neurotic.|
|3||PATHOLOGICAL. This character has become openly destructive to himself and/or others. He has only the barest, most tenuous connection with reality.|
|TOTALLY FRAGGING BONKERS|
|2 or less||PSYCHOTIC. This character has cracked beyond hope of recovery. His psychological problems are compounded upon one another. Any defining traits are present to the most extreme extent possible.|
Every time a character's Essence changes, his Sanity changes with it. Karma boosts to Sanity are permanent, and the Sanity is recomputed with those points added on. Since Sanity varies arithmetically with ESS/2, it is easiest simply to deduct half the points from a character's Sanity that one does from his Essence.
It does follow, incidentally, that a character under an Increase Charisma or Increase Willpower spell will find himself capable of acting more sanely than he otherwise might. As long as the spell is in effect, the character's Sanity will be artificially raised. (The corollary is that a Decrease Charisma or Decrease Willpower spell can make a character come apart at the seams.)
Note how loss of Essence can demolish one's Sanity in a hurry. Be judicious.
Big Ethel is a Troll Street Samurai with Charisma 2, Willpower 2 and Essence 1.2. Her Sanity is 2 + (2 + 1.2) / 2 = 3.6. This places her in the Pathological category. Trolls are notoriously brutal types, so Big Ethel's player makes her a violently destructive megalomaniac. Good luck, world.
Charlie X is a Government Agent with Charisma 3, Willpower 4 and Essence 3.4. His Sanity is 3 + (4 + 3.4) / 2 = 6.7. This places him in the Offensive category. Of all the possible ways an agent could be offensive, Charlie's player decides that good, insufferable self-righteousness seems well suited to a G-man. Charlie X is a judgmental perfectionist, deathly afraid of being caught in an ethical lapse himself. Two Karma points would raise his Sanity to 7.1, making him merely Defensive - perhaps just a bit stiff and uptight.