[en] Kepler-454 (KOI-273) is a relatively bright (V = 11.69 mag), Sun-like star that hosts a transiting planet candidate in a 10.6 day orbit. From spectroscopy, we estimate the stellar temperature to be 5687 ± 50 K, its metallicity to be [m/H] = 0.32 ± 0.08, and the projected rotational velocity to be v sin i < 2.4 km s⁻¹. We combine these values with a study of the asteroseismic frequencies from short cadence Kepler data to estimate the stellar mass to be ${1.028}_{-<!--\; ???\; -->0.03}^{+0.04}{M}_{\odot <!--\; ???\; -->}$, the radius to be 1.066 ± 0.012 R_⊙, and the age to be ${5.25}_{-<!--\; ???\; -->1.39}^{+1.41}$ Gyr. We estimate the radius of the 10.6 day planet as 2.37 ± 0.13 R_⊕. Using 63 radial velocity observations obtained with the HARPS-N spectrograph on the Telescopio Nazionale Galileo and 36 observations made with the HIRES spectrograph at the Keck Observatory, we measure the mass of this planet to be 6.8 ± 1.4 M_⊕. We also detect two additional non-transiting companions, a planet with a minimum mass of 4.46 ± 0.12 M_J in a nearly circular 524 day orbit and a massive companion with a period >10 years and mass >12.1 M_J. The 12 exoplanets with radii <2.7 R_⊕ and precise mass measurements appear to fall into two populations, with those <1.6 R_⊕ following an Earth-like composition curve and larger planets requiring a significant fraction of volatiles. With a density of 2.76 ± 0.73 g cm⁻³, Kepler-454b lies near the mass transition between these two populations and requires the presence of volatiles and/or H/He gas.