The stamina has many purposes, it can be used to jump, to levitate, or to protect. The stamina can decrease quickly, but it also increases quickly too.
These are images of stamina being used:[]
Rena doing a short jump, this uses less of the stamina bar
Rena doing a high jump, using up the stamina bar
Rena levitating
Rena leaping forward
Rena dodging to the left
Rena blocking
Rena blocking against an attack with almost no stamina left