Mental Fortitude

Mental Fortitude describes how strong-willed a person is when involved in a Psychic Struggle. A stronger mental fortitude makes it more likely to deliver or resist mental status effects, such as causing someone to become irritated, insane, tired or enraged using magic.

Increased power in Psychic struggles
A Psychic Struggle involves an opposed attempt to inflict upon / defend against a mental status effect, such as being tired or enraged. Having more mental fortitude allows greater resistance to things such as going insane. This can be best seen as grit, the courage or resolve to fight off a specific status effect or dampen its effectiveness.

Increased resilience and bravery
Having more mental fortitude makes Fear & Demoralization, Gaslighting and Soul Break tactics less effective in Combat.

Examples
An extreme example of this is Happy, who due to his inability to feel, is so resilient to psychic struggles that he is almost completely immune, which surpasses Katie in mental fortitude. He is also not afraid of death due to his already suicidal tendencies, and combined with the Death Protection that Katie gave, Happy is not going to be scared, fooled by Gaslighting - and it is very difficult to Soul Break him due to not having much of one. Due to this, he is a direct counter to Nightmare's main method of attack.

See Magic Stats for more stats that are inherent to a magic user's mind.