Bathing with people in the pool is something you can do throughout the game, but to unlock this trophy you will need to raise your friendship with a character to the level of "soulmate". Once you've done that, when you enter the pool you will have the option to bring them along directly instead of bathing with someone random.
This will probably happen naturally at some point, but you can raise a character's affinity by taking them on missions, completing quests for them, or if you happen to meet them in the pool. You will also advance your relationship with certain characters as part of the story. You can check how you're doing by talking to Shusui.