  We present a detailed analysis of the XMM-Newton observations of five
narrow-line Seyfert 1 galaxies (NLS1s). They all show very soft continuum
emission in the X-ray band with a photon index of $\Gamma\gtrsim 2.5$.
Therefore, they are referred to as `ultra-soft' NLS1s in this paper. By
modeling their optical/UV-X-ray spectral energy distribution (SED) with a
reflection-based model, we find indications that the disc surface in these
ultra-soft NLS1s is in a higher ionisation state than other typical Seyfert 1
AGN. Our best-fit SED models suggest that these five ultra-soft NLS1s have an
Eddington ratio of $\lambda_{\rm Edd}=1-20$ assuming available black hole mass
measurements. In addition, our models infer that a significant fraction of the
disc energy in these ultra-soft NLS1s is radiated away in the form of
non-thermal emission instead of the thermal emission from the disc. Due to
their extreme properties, X-ray observations of these sources in the iron band
are particularly challenging. Future observations, e.g. from Athena, will
enable us to have a clearer view of the spectral shape in the iron band and
thus distinguish the reflection model from other interpretations of their
broadband spectra.
