  Existing analysis based on XMM-Newton/RGS spectra already shows that the
G-ratio of the OVII He$\alpha$ triplet in the inner bulge of M31 is too high to
be consistent with a pure optically thin thermal plasma in collisional
ionization equilibrium (CIE). Different processes that may affect properties of
diffuse hot plasma were proposed, such as resonance scattering (RS) and charge
exchange (CX) with cold gas. To determine which physical process(es) may be
responsible for this inconsistency, we present a systematic spectroscopic
analysis based on 0.8 Ms XMM-Newton/RGS data, together with complementary
Chandra/ACIS-S images. The combination of these data enables us to reveal
multiple non-CIE spectroscopic diagnostics, including but not limited to the
large G-ratios of He$\alpha$ triplets (OVII, NVI, and NeIX) and the high Lyman
series line ratios (OVIII Ly$\beta$/Ly$\alpha$ and Ly$\gamma$/Ly$\alpha$, and
NVII Ly$\beta$/Ly$\alpha$), which are not expected for a CIE plasma, and the
high iron line ratios (FeXVIII 14.2 \AA/FeXVII~17 \AA and FeXVII~15 \AA/17
\AA), which suggest much higher temperatures than other line ratios, as well as
their spatial variations. Neither CX nor RS explains all these spectroscopic
diagnostics satisfactorily. Alternatively, we find that an active galactic
nucleus (AGN) relic scenario provides a plausible explanation for virtually all
the signatures. We estimate that an AGN was present at the center of M31 about
half a million years ago and that the initial ionization parameter $\xi$ of the
relic plasma is in the range of 3-4.
