Discoveries of billion-solar-mass black holes within the first 10% of the Universe's age have put a stringent constraint on their formation and early growth history. Recent observational efforts have not only pushed the redshift limit to z=7.6, but also probed the quasar population down to the faint end, i.e., a general supermassive BH population in the early universe. In this talk, I will start by showing how the high-redshift quasars are characterized by observations, covering a various topics such as BH mass, chemical enrichment of broad-line-region gas, and cold gas dynamics of host galaxies. The second-half of this talk focuses on a Subaru/HSC project that has revealed more than 100 low-luminosity quasars at z>6. I will also introduce our approved Cy1 JWST observations, in which we aim to uncover the BH mass distribution and the BH-to-stellar mass ratios at z=6.