  The light curve of the microlensing event KMT-2021-BLG-0912 exhibits a very
short anomaly relative to a single-lens single-source form. We investigate the
light curve for the purpose of identifying the origin of the anomaly. We model
the light curve under various interpretations. From this, we find four
solutions, in which three solutions are found under the assumption that the
lens is composed of two masses (2L1S models), and the other solution is found
under the assumption that the source is comprised of a binary-star system (1L2S
model). The 1L2S model is ruled out based on the contradiction that the faint
source companion is bigger than its primary, and one of the 2L1S solutions is
excluded from the combination of the relatively worse fit, blending constraint,
and lower overall probability, leaving two surviving solutions with the
planet/host mass ratios of $q\sim 2.8\times 10^{-5}$ and $\sim 1.1\times
10^{-5}$. A subtle central deviation supports the possibility of a tertiary
lens component, either a binary companion to the host with a very large or
small separation or a second planet lying near the Einstein ring, but it is
difficult to claim a secure detection due to the marginal fit improvement, lack
of consistency among different data sets, and difficulty in uniquely specifying
the nature of the tertiary component. With the observables of the event, it is
estimated that the masses of the planet and host are $\sim (6.9~M_\oplus,
0.75~M_\odot)$ according to one solution and $\sim (2.8~M_\oplus,
0.80~M_\odot)$ according to the other solution, indicating that the planet is a
super Earth around a K-type star, regardless of the solution.
