  The exotic $^9$He nucleus, which presents one of the most extreme
neutron-to-proton ratios, belongs to the $N=7$ isotonic chain famous for the
phenomenon of ground-state parity inversion with decreasing number of protons.
Consequently, it would be expected to have an unnatural (positive) parity
ground state similar to $^{11}$Be and $^{10}$Li. Despite many experimental and
theoretical investigations, its structure remains uncertain. Apart from the
fact that it is unbound, other properties including the spin and parity of its
ground state and the very existence of additional low-lying resonances are
still a matter of debate. In this work we study the properties of $^9$He by
analyzing the $n+^8$He continuum in the context of the ab initio no-core shell
model with continuum (NCSMC) formalism with chiral interactions as the only
input. The NCSMC is a state-of-the-art approach for the ab initio description
of light nuclei. With its capability to predict properties of bound states,
resonances, and scattering states in a unified framework, the method is
particularly well suited for the study of unbound nuclei such as $^9$He. Our
analysis produces an unbound $^9$He nucleus. Two resonant states are found at
the energies of ${\sim}1$ and ${\sim}3.5$ MeV, respectively, above the $n+^8$He
breakup threshold. The first state has a spin-parity assignment of $J^{\pi} =
{1/2}^-$ and can be associated with the ground state of $^9$He, while the
second, broader state has a spin-parity of ${3/2}^-$. No resonance is found in
the ${1/2}^+$ channel, only a very weak attraction. We find that the $^9$He
ground-state resonance has a negative parity and thus breaks the
parity-inversion mechanism found in the $^{11}$Be and $^{10}$Li nuclei of the
same $N=7$ isotonic chain.
