  Hydrogen lines from forming planets are crucial for understanding planet
formation. However, the number of planetary hydrogen line detections is still
limited. Recent JWST/NIRSpec observations have detected Paschen and Brackett
hydrogen lines at TWA 27 B (2M1207b). TWA 27 B is classified as a
planetary-mass companison (PMC) rather than a planet due to its large mass
ratio to the central object ($\approx 5 M_\mathrm{J}$ compared to $25
M_\mathrm{J}$). Nevertheless, TWA 27 B's hydrogen line emission is expected to
be same as for planets, given its small mass. We aim to constrain the accretion
properties and accretion geometry of TWA 27 B, contributing to our
understanding of hydrogen line emission mechanism common to both PMCs and
planets. We conduct spectral fitting of four bright hydrogen lines
(Pa-$\alpha$, Pa-$\beta$, Pa-$\gamma$, Pa-$\delta$) with an accretion-shock
emission model tailored for forming planets. We estimate the mass accretion
rate at $\dot{M} \approx 3 \times 10^{-9}\, M_\mathrm{J}\,\mathrm{yr}^{-1}$
with our fiducial parameters, though this is subject to an uncertainty of up to
factor of ten. Our analysis also indicates a dense accretion flow, $n\gtrsim
10^{13}\,\mathrm{cm^{-3}}$ just before the shock, implying a small
accretion-shock filling factor $f_\mathrm{f}$ on the planetary surface
($f_\mathrm{f} \lesssim 5\times10^{-4}$). This finding suggests that
magnetospheric accretion is occurring at TWA 27 B. Additionally, we carry out a
comparative analysis of hydrogen-line emission color to identify the emission
mechanism, but the associated uncertainties proved too large for definitive
conclusions. This underscores the need for further high-precision observational
studies to elucidate these emission mechanisms fully.
