Kashirin is a russian political operative and the captain of the warship, Balashevo, which was docked in Tokyo Harbour and secretly armed with a nuclear missile launch control centre.
When Godzilla submerged in Tokyo Bay, damaging the Balashevo, the weapons systems malfunctioned and began automatically counting down to fire the Soviet Nuclear Attack Satellite. Kashirin, injured from being thrown to the side of the boat when it was hit by the submersion wave, bravely forced his way through a damaged doorway to try and stop the countdown, but was killed by a small explosion.
In the U.S. version of The Return of Godzilla, Kashirin deliberately sets off the missile. The Balashevo isn't hit with a wave, and thus he survives, though his fate is left unknown. In this version, Kashirin's actions indirectly lead to the revival of Godzilla, with the kaiju becoming revived by the nuclear storms when the missile explodes.