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RX J0134.2-4258 is one of the most super-Eddington narrow-line Seyfert 1 (NLS1) galaxies, on which we conducted a monitoring campaign from radio to X-rays. In this paper, we present a detailed analysis of its optical/UV spectra and broadband spectral energy distribution (SED). Our study shows that the preferred black hole mass of RX J0134.2-4258 is $M_{\rm BH} \sim 2 \times 10^{7}~M_{\odot}$, giving a mass accretion rate through the outer disc of $\dot{m}_{\rm out} \sim 20$ (assuming zero spin), compared to the observed luminosity ratio $L_{\rm bol}/L_{\rm Edd} \sim 6$. This reduction in radiative efficiency is expected for super-Eddington flows, as power can be lost via advection and/or disc winds. We find that the optical/UV lines of RX J0134.2-4258 resemble those from weak-like quasars (WLQs), as it has notably weak C IV and N V emission lines. It also has drastic X-ray variability, again similar to that recently observed in some other WLQs. However, WLQs have systematically higher masses ($\gtrsim 10^8~M_{\odot}$), and lower Eddington ratios ($\dot{m}_{\rm out} \sim 1$) than RX J0134.2-4258. We compare instead to the most extreme NLS1s, with similarly large $\dot{m}_{\rm out}$ but smaller masses. These show similarly large reductions in radiative efficiency but their UV lines are not similarly wind-dominated. We suggest a new category of weak-line Seyfert (WLS) galaxies to describe sources like RX J0134.2-4258, and interpret its (so far unique) properties in a model, where the lower-disc-temperature in the higher-mass black holes leads to the UV-line-driving mechanism, which enhances the super-Eddington radiation-pressure-driven wind.