  We present a comprehensive spectral analysis of the ultraluminous X-ray
source Holmberg II X-1 using broadband and high-resolution X-ray spectra taken
with the XMM-Newton satellite over a period of 19 years benefiting from a
recent campaign. We tested several models for the broadband spectra among which
a double thermal component provided a reasonable description for the continuum
between 0.3-10 keV and enabled us to constrain the properties of the accretion
disc. The Luminosity-Temperature trends of the inner and outer disc components
broadly agree with the expectations for a thin disc, although the exact values
of the slopes are slightly sensitive to the adopted model. However, all tested
models show L-T trends which deviate from a power law above a bolometric
luminosity of about 5 $\times \ 10^{39} $erg/s, particularly for the hot
thermal component associated to the inner accretion flow. Assuming that such
deviations are due to the accretion rate exceeding its Eddington limit or, most
likely, the super-critical rate, a compact object with a mass 16-36 Msun, i.e.
a stellar-mass black hole, is inferred. The time-averaged (2021) high
resolution spectra present narrow emission lines at 1 keV primarily from Ne
IX-X and a very strong at 0.5 keV from N VII, which indicate Ne-N-rich gas with
non-Solar abundances. This favours a nitrogen-rich donor star, such as a
blue/red supergiant, which has escaped from its native stellar cluster
characterised by a low-metallicity environment.
