Intelligence is one of the four main attributes available in the Geneforge Series that can be increased. Intelligence affects the amount of Essence and Spell Energy the PC has. In Geneforge 1 it cost 5 Skill Points to increase, while subsequent Geneforge versions had requirements increasing for every level advanced.
In creations, this only affects the amount of energy they have, as essence is not included for them. Intelligence also increases the resistance of mental effects. This can be useful for creations such as the Thahd as they can be charmed or dazed with little effort. The first two points of intelligence allows the user to control the creation; otherwise the AI will be turned on for it.
These are recommendations for each class.
Intelligence is essential for these two classes. Shapers will need as much Essence as possible to maximize their creations' potential. Some essence should be saved for extra buffing magic, especially Blessing Magic.
Intelligence is essential for these two classes. Agents require a huge amount of Spell Energy and Essence to stay in combat for extended periods of time. In Geneforge 1 maximize this as much as possible (around level 15-20 at the least) to ensure that the Agent can continuously cast Essence Orbs. In the later Geneforge versions, upgrading to around where the cost per level is 8-9 skill points is sufficient. However, huge energy-consuming spells such as Kill and Aura of Flames will drain an Agent after several casts even with high intelligence in high levels.
Intelligence is somewhat essential for these two classes. Guardians are not weak at shaping and definitely require some creations to back them up. The mental resistance is also useful.