Being part of a binary system, Manwë has one known companion named Thorondor, formally designated (385446) Manwë I Thorondor. It is estimated to be about two-thirds the size of the primary, approximately for a volume equivalent diameter. The rotation period of Thorondor is uncertain, though a best-fit model suggests a very slow rotation period of 309.3 days. Thorondor's rotation is expected to be chaotic like Pluto's smaller moons, as a result of gravitational torquing by Manwë over the course of their eccentric mutual orbit. Thorondor's light curve has considerable photometric variability, with the relative magnitude of the two objects measured variously from 0.6–2.1 over the course of a few years. This implies that Thorondor could have a very flattened shape, akin to the larger lobe of the contact binary Kuiper belt object 486958 Arrokoth. Assuming a ellipsoid shape for Thorondor, a best-fit model for its shape suggests the aspect ratios of a/c = 7.33 and b/c = 6.67.
