A dust-reddened quasar in a [C II]-luminous host at z=6.55

Most known quasars at z>6 were selected through their rest-frame UV emission. Only a small number of reddened quasars have been identified at these redshifts, leaving their nuclear and host-galaxy properties poorly constrained. We present JWST/NIRSpec spectroscopy and ALMA band 6 observations of the quasar J1440+0019 at z=6.5488, revealing its dust-reddened nature and jointly characterizing its nucleus and host galaxy. The continuum is best described by an SMC-like extinction law with E(B-V)=0.20 plus or minus0.03. Three absorption systems detected in N V, Si IV, and C IV are blueshifted by approximately 900 -- 3600 km s-1 and likely trace an ionized nuclear outflow. The broad Mg II line and extinction-corrected continuum yield a black-hole mass of (M BH/M)=9.04 plus or minus0.42, a bolometric luminosity of L bol 1.1 times10 47 erg s-1, and an Eddington ratio of λ Edd 0.8. ALMA observations further reveal its host galaxy to be among the most [C II]-luminous known at z>6, with L[C II]=(7.21 plus or minus0.25) times10 9 L. For a fiducial dust temperature of 47 K and emissivity index β=1.6, the dust continuum implies L TIR=8.9 times10 12 L and M dust=4.0 times10 8 M. The inferred dynamical mass places J1440+0019 in the region of the M BH -- M dyn plane occupied by high-redshift blue quasars. These observations reveal a moderately reddened, rapidly accreting quasar in a luminous, dust-rich host during reionization. Its measured nuclear and host-galaxy properties provide an observational reference for identifying and characterizing similar systems in future systematic searches with wide-area infrared surveys.