Guide is Ophilia's Path Action in Octopath Traveler. This ability allows Ophilia to guide and escort certain non-playable characters in the game, and lead to them to various locations. This Path Action is the noble counterpart to Primrose's Path Action, Allure.
When choosing to guide an NPC, you will only be allowed to do so if Ophilia is a certain level or greater. The more powerful the NPC is in battle, the higher the level you need to be. If the action is successful, the NPC will begin to follow Ophilia and will walk behind her.
When in combat, a 'Support' option becomes available. When used, the guided NPC enters the battle and every turn, after Ophilia's turn, will attack with either their standard attack or their special skill. When the enemy attacks, they have a chance to attack the support instead of your characters. After a given amount of attack turns or upon defeat, they will leave the battle, but will continue to follow the player. Each NPC can only be summoned into battle a certain number of times (varying between NPCs), at which point, they can no longer be summoned.
Outside of combat, this path action can also be used to progress the plot or certain side quests by leading certain NPCs to certain locations (such as reuniting a lost boy with his mother). If at any point the player does not want the NPC by their side anymore, they can press 'Y' to part ways with them. Ophilia can have only one follower at any given time. If you try to guide a second, you'll have to part ways with your current follower.