  Superstring compactification on a manifold of Spin(7) holonomy gives rise to
a 2d worldsheet conformal field theory with an extended supersymmetry algebra.
The $\mathcal{N}=1$ superconformal algebra is extended by additional generators
of spins 2 and 5/2, and instead of just superconformal symmetry one has a
$c=12$ realization of the symmetry group $\mathcal{SW}(3/2,2)$. In this paper,
we compute the characters of this supergroup, and decompose the elliptic genus
of a general Spin(7) compactification in terms of these characters. We find
suggestive relations to various sporadic groups, which are made more precise in
a companion paper.
