Guard is a function usable by all player characters. Its function is defensive. Any time a player guards, they are healed for 10% of their total health and any attacks to the player that follow will deal half damage. The Guard function is particularly useful for keeping at least one person alive during a boss fight, or healing under desperate circumstances. But, the best advantage from guard is the character will immune against any ailments.
Arm-blocking, poison, fear, and other ailment effects tend to slow battles down, by requiring more healing, or use of recovery items, or reducing player damage; although such a use is time-consuming, and may counterindicated, Guard can be used instead of Body Purifiers or Mind Purifiers to let such effects run their course (usually three rounds).
Although such a use is time-consuming, Guard can also be used to give two characters or even one single character more opportunities to perform their Additions, thus requiring less time (running around in circles on the World Map, in all likelihood) searching for battles in order to level up these useful and fun skills.