This involves having two characters that hate each other (opposites).
Explanation for campfire stories, ignore if you understand the system:
*** Spoiler - click to reveal ***This is done through the campfire system where once per expedition in the bottom right corner of the screen the campfire bar fills up as you progress. When selecting the campfire you will have the choice of four randomly selected cards from a pool determined by the crew you have selected. The unique cards for each crew member must first be unlocked by completing adventures to aquire booster packs. The gold coloured personal campfire stories are the easiest way to build relationships between your crew.
The easiest crew I found to get this achievement involves Hojo Fumiaki (scout) and Wang Yu (fighter). It is important to select the personal campfire stories in early expeditions as the relationships will take subsequent expeditions to build. Save scumming may be used to ensure you get the campfire stories you need, typically saving a node before the campfire is available.
Personal campfire stories:
Wang Yu: Non conditional, after subsequent expeditions both other crew members will become opposites of Wang.
Hojo: Select Wang as the targeted crew member. After subsequent expeditions Wang will become Hojo's opposite IF Wang is not level three or higher in Quick Thinker, Naturalist and Tactician.
Third crew member: There shouldn't be a member that can intefere with Wang and Hojo's relationship. If struggling to save scum, Kiwi and Yvonne's stories have the ability for a targeted crew member to become opposites.
Wiki links: