The Head of Alcasan
The N.I.C.E. is led and controlled by the leadership of the disembodied head of Alcasan, a recently executed French scientist who has been resurrected by dark means. He is now controlled, it seems, by some demon, most likely Satan himself. This twisted, unnatural, horrifying head causes disgust in its observers; it is a symbol of the true nature of evil - reviling, power-hungry, and despicable. The fact that Alcasan is only a head, without any powers to move or enact any actions, also implies that evil has no power to harm God's people.
Merlinus Ambrosius
The significance of including the sorcerer Merlin in this story is intentional on the part of Lewis. It is tempting to merely see him as a coincidentally present, powerful being who can turn the tide of the war. Lewis, however, had a deeper meaning for writing him into the story. Lewis was interested in pagan mythology and how it crosses over with Christianity, influencing most of his fiction, such as the Chronicles of Narnia (fauns and centaurs) and Till We Have Faces (based on the myth of Cupid and Psyche). By incorporating a character from Arthurian legend into That Hideous Strength, Lewis is demonstrating how the good and fantastic elements of mythos can be reconciled with Christianity, arguing that simply because it's pagan does not mean it isn't worthwhile to study and enjoy.
Grace Ironwood
The character of Grace Ironwood further serves to heighten Lewis's point about the reconciliation of myth and Christianity. She is stern yet compassionate, mirroring her two names: Ironwood (harsh, reminiscent of the Iron-Wood of Norse mythology) and Grace (a Christian name that redeems the pagan qualities of the Iron Wood). Like Merlin, her actions and qualities demonstrate a marriage of Christianity with pagan mythology, showing the worthiness of the latter in the light of the former - unsurprising, since Lewis was a professional Christian classicist.
St. Anne's
St. Anne's is the home base, so to speak, of the forces of good on earth. It is the site of Dr. Ransom's organization, who, in league with the Oyéresu of several planets, launch an attack on the N.I.C.E. and finally defeat it. The name isn't accidental; St. Anne's is reminiscent of a Catholic or Anglican church, and the church symbolism doesn't stop there. The church should be the home of Christians and the organization through which they may be strengthened and sent out to bring light to the darkness. St. Anne's, in this novel, therefore represents the ideal function of the church in today's society.
Dr. Ransom (The Director)
In the previous novels, Dr. Elwin Ransom has functioned as a sort of Christ-figure, sacrificing himself for the sake of others and defeating the forces of evil. The symbolism is heightened in this novel: he leads the forces of goodness into battle against the evils of N.I.C.E., and his new title (Pendragon) indicates that he has assumed his long-coming title of King. The wound in his heel, received during the events of Perelandra, implies his Christhood as well; it reflects the prophecy in Genesis that the serpent (Satan) will strike the heel of Christ, mankind's redeemer. Ransom also adopts the last name Fisher-King, which is the title of a heroic king from Arthurian myth who bears many similarities to the character of Christ.
