For each finite, irreducible Coxeter system (W, S), Lusztig has associated a set of "unipotent characters" Uch(W ). There is also a notion of a "Fourier transform" on the space of functions Uch(W ) → R, due to Lusztig for Weyl groups and to Broué, Lusztig, and Malle in the remaining cases. This paper concerns a certain W -representation ̺ W in the vector space generated by the involutions of W . Our main result is to show that the irreducible multiplicities of ̺ W are given by the Fourier transform of a unique function ǫ : Uch(W ) → {−1, 0, 1}, which for various reasons serves naturally as a heuristic definition of the Frobenius-Schur indicator on Uch(W ). The formula we obtain for ǫ extends prior work of Casselman, Kottwitz, Lusztig, and Vogan addressing the case in which W is a Weyl group. We include in addition a succinct description of the irreducible decomposition of ̺ W derived by Kottwitz when (W, S) is classical, and prove that ̺ W defines a Gelfand model if and only if (W, S) has type A n , H 3 , or I 2 (m) with m odd. We show finally that a conjecture of Kottwitz connecting the decomposition of ̺ W to the left cells of W holds in all non-crystallographic types, and observe that a weaker form of Kottwitz's conjecture holds in general. In giving these results, we carefully survey the construction and notable properties of the set Uch(W ) and its attached Fourier transform.