Upper panel: combined LBT/MODS and LBT/LUCI spectra of J1641, smoothed with a Gaussian function with a standard deviation equal to the instrument wavelength resolutions (black line). We also show the composite spectrum of z > 5.7 QSOs of Shen et al. (2019, orange line), renormalized at rest-frame 1700 Å. The expected locations of emission lines at the measured redshift of J1641+3755 (i.e., 6.025) are marked as vertical dashed lines. The dashed violet line represents the best-fitting power-law continuum. Several narrow absorption lines are found in the spectrum: their locations are marked with short vertical ticks (see Table 5 and Fig. 6). We identified some of the absorption features with transitions due to an intervening system at z ≈ 5.67 (red ticks), while others are currently unidentified (gray ticks). Lower panel: error of the spectrum.