We study the half-advanced and half-retarded Wheeler Green function and its
relation to Feynman propagators, first for massless equation, then, for Kl
ein-Gordon equations with arbitrary mass parameters, real, imaginary, or co
mplex. In all cases the Wheeler propagator lacks an on-shell free propagati
on. The Wheeler function has support inside the light-cone (whatever the ma
ss). The associated vacuum is symmetric with respect to annihilation and cr
eation operators. We show with some examples that perturbative unitarity ho
lds, whatever the mass (real or complex). Some possible applications are di
scussed.