  We introduce on-shell variables for Heavy Particle Effective Theories (HPETs)
with the goal of extending Heavy Black Hole Effective Theory to higher spins
and of facilitating its application to higher post-Minkowskian orders. These
variables inherit the separation of spinless and spin-inclusive effects from
the HPET fields, resulting in an explicit spin-multipole expansion of the
three-point amplitude for any spin. By matching amplitudes expressed using the
on-shell HPET variables to those derived from the one-particle effective
action, we find that the spin-multipole expansion of a heavy spin-$s$ particle
corresponds exactly to the multipole expansion (up to order $2s$) of a Kerr
black hole, that is, without needing to take the infinite spin limit. Finally,
we show that tree-level radiative processes with same-helicity bosons emitted
from a heavy spin-$s$ particle exhibit a spin-multipole universality.
