Protein can be used on a specific party member to restore 30% of their HP. The item can only be used outside of battle.
If the protagonisttrains at Café Leblanc or at the Protein Lovers Gym in Shibuya, he'll have the option to use one Protein for the activity. If used while training at Leblanc, he'll gain +1 HP at the end of the activity. If at the gym, +2 HP and +1 SP.