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We present optical spectroscopic and milli-arcsecond scale radio continuumobservations of the quasar M1540-1453 ($z_{em}$ = 2.104$\pm$0.002) that showsassociated HI 21-cm absorption at $z_{abs}$ = 2.1139. At sub-kpc scales, thepowerful radio source with 1.4 GHz luminosity of $5.9\times10^{27}$ WHz$^{-1}$shows Fanaroff-Riley (FR) class I morphology caused by the interaction withdense gas within 70 pc from the AGN. Interestingly, while there are indicationsfor the presence of absorption from low-ionization species like FeII, SiII andSiIII in the optical spectrum, the expected strong damped Ly$\alpha$ absorptionis not detected at the redshift of the HI 21-cm absorber. In comparison totypical high-$z$ quasars, the Ly$\alpha$ emission line is much narrower. The`ghostly' nature of the HI Ly$\alpha$ absorber partially covering the broadline region of extent 0.05 pc and the detection of widespread HI 21-cmabsorption covering the diffuse radio source (extent $>$425 pc) imply thepresence of a large clumpy HI halo -- which may have been blown by the jet-ISMinteraction. Further observations are needed to confirm the `ghostly' nature ofthe Ly$\alpha$ absorber, and obtain a better understanding of the role playedby the jet-ISM interaction in shaping the radio morphology of this powerfulAGN. The study showcases how joint radio and optical analysis can shed light ongaseous environment and origin of radio morphology in AGN at high redshifts,when these are still the assembly sites of giant galaxies.

HI gas playing hide-and-seek around a powerful FRI-type quasar at z$\sim$2.1