  KAGRA is a newly built gravitational wave observatory, a laser interferometer
with a 3 km arm length, located in Kamioka, Gifu, Japan. In this series of
articles, we present an overview of the baseline KAGRA, for which we finished
installing the designed configuration in 2019. This article describes the
method of calibration (CAL) used for reconstructing gravitational wave signals
from the detector outputs, as well as the characterization of the detector
(DET). We also review the physical environmental monitors (PEM) system and the
geophysics interferometer (GIF). Both are used for characterizing and
evaluating the data quality of the gravitational wave channel. They play
important roles in utilizing the detector output for gravitational wave
searches. These characterization investigations will be even more important in
the near future, once gravitational wave detection has been achieved, and in
using KAGRA in the gravitational wave astronomy era.
