Understanding the interstellar medium (ISM) is the first step to understanding star formation/evolution and, ultimately, planet formation. The formation of molecular gas occurs in diffuse clouds. These clouds are also closely associated with the progenitors of molecular clouds and clumps, which are the potential birthplace of stars. Not only that, but also the composition of diffuse clouds is essential to the chemical evolution of molecular clouds since their chemical condition can reflect the initial chemical condition of molecular clouds. The initial chemical condition is crucial to explain derived quantities for observed molecular species toward star-forming molecular clouds. To characterize the properties of the Galactic ISM, the HyGAL Stratospheric Observatory for Infrared Astronomy legacy program surveys six hydride molecules (ArH+, OH+, H2O+, SH, OH, and CH) and two atomic constituents (C+ and O) within the diffuse interstellar medium (ISM) by observing absorption lines toward 25 bright continuum Galactic sources. As ancillary data, we also used absorption lines of simple molecules (HCO+, HCN, HNC, C2H, c-C3H2, CS, and H2S) obtained with the IRAM 30m telescope. A principal component analysis (PCA) of the column densities of all the species mentioned above classifies three species groups that show strong correlations or weak correlations to other groups among themselves (Group 1: C+ and hydride ions, group 2: neutral molecules and HCO+ and group 3: CH, OH, and O). Based on the PCA results, the neutral hydrides and O lines significantly prefer to trace diffuse cloud components with intermediate conditions between high atomic gas- and high molecular gas-fraction diffuse ISM. We will present detailed comparisons of atomic O, OH, and CH absorption lines and variations in their abundance and column density, depending on the surrounding environment (e.g., star formation activities or supernova remnants).