Though not part of the Iliads epic triangle, Thetis plays a key role as Achilles’ divine parent, confidante, and source of prophecy. In much of this she looks back, to Ninsun in Gilgamesh, and ahead, to Venus in the Aeneid. In the single most distinctive feature of the Aeneid's divine economy, Virgil essentially makes Venus part of the epic triangle and dispenses with Minerva; we will consider this below. First, Ninsun: all of Thetis’ central functions are predicted by Ninsun’s role in Gilgamesh. Gilgamesh’s divine mother, the goddess Ninsun, meets with him (I.5-6, III.1-4), discusses his impending encounters with Enkidu and Humbaba, and worries about his risking his life (III.2-5). As a goddess, she also interprets his dreams, a counterpart to Thetis' relaying prophecies to Achilles and warning him about the implications ofhis duel with Hektor. Thetis' prominence, a further Iliadic complication beyond the traditional triangle, partly obscures Athena's more typical epic role as Achilles' mentor. There is some overlap in Thetis and Athena's dual roles as Achilles' confidante (though even Hera and Poseidon briefly have this function), but the poem carefully signals Athena's primary role as mentor by giving her the first theophany (1.194), and employing a climactic instance of the same when she oversees Achilles’ slaying of Hektor (22.214). (On the role of divine mothers in epic poetry, see Chapter 8, by H. Foley.)
Thetis' functions in the Iliad suggest broad structural parallels with Calypso and Circe in the Odyssey. All are goddesses outside the Olympic pantheon who have intimate interactions with the hero, but are less involved with his actions as a warrior than is typical of deities such as Athena or Apollo. But in the thematic way of epic the Odyssey’s presentation of Circe and Calypso is affected by plot concerns specific to the Odyssey. The poem has a thrice-repeated sequence of Odysseus landing on an island and carefully approaching and negotiating with a powerful female figure who holds power over his access to the next stage of his homecoming (for fuller discussion see Louden 1999: 4-14, 104-29; see also Chapter 8, by H. Foley on women in epic generally). The Odyssey’s presentation of Circe is largely shaped by this overriding concern. She is one of a series of females (including his wife, Penelope, and the Phaeacian queen Arete), who impose tests on Odysseus, and, when he passes their tests, help him advance ever closer to his nostos, or homecoming. Calypso, on the other hand, though readers often lump her together with Circe, is presented as a deliberate antitype to Circe (and to Penelope and Arete), a female who desires Odysseus without imposing tests upon him, who does not want to help him leave. Odysseus has sex with both Calypso and Circe, but again, with almost opposite effect. The Odyssey alludes to a lengthy sexual relationship between Odysseus and Calypso, but to only one occasion when Odysseus and Circe have sex. When Odysseus wishes to leave Calypso attempts to detain him, but Circe immediately tells him how to leave. Thetis' sexuality, meanwhile, is treated very differently, but does indirectly affect the protagonist, Achilles. The Iliad merely alludes to Thetis’ sexual desirability to Zeus, instead presenting his acquiesence to her request as dependent upon her role in an earlier, serious, theomachy (1.396-406). Calypso and Thetis share an additional feature: both have an important blocking effect on the overall plot; each helps to remove the protagonist from the main storyline, thus stalling its progression in his absence. When Thetis gets Zeus to accede to Achilles’ request, it results in his being essentially missing from action in Books 2-17. Calypso likewise keeps Odysseus on her remote isle for seven years.