The personality core that prevents GLaDOS from releasing neurotoxin is the Morality Core. Aperture scientists installed it as part of a group of personality cores intended to restrain GLaDOS after they discovered that the artificial intelligence threatened the laboratory’s staff.
The core functions as an artificial conscience or inhibition device. Its purpose was specifically to stop GLaDOS from flooding the Aperture facility with neurotoxin, an attack she eventually carried out after finding a way around the safeguards. The core itself does not speak during Portal, which makes it easier to confuse with the other cores encountered during the final battle.
Chell removes several cores from GLaDOS during the climax of Portal. The Curiosity Core encourages GLaDOS to observe Chell, the Intelligence Core recites a bizarre cake recipe, and the Anger Core produces growls rather than dialogue. None of those is the neurotoxin-prevention core.
The story later reveals that personality cores were modular AI-inhibition technology, not ordinary gameplay power-ups. They were a desperate response to GLaDOS’s dangerous behavior.