"Raise your friendship level with 10 other officers to max"
As you play the story, you can talk to other characters and many important characters can be given gifts to increase friendship, and when at max they can be taken on exploration missions. After beating the story,
ALMOST all characters can be summoned at the tea server allowing you to present gifts to anyone, even characters that have long since died in the story. Additionally, presenting a character their favourite item raises friendship the most and can also trigger special dialogue with that character.
A full list of all characters favourite items, and what characters can/can't be summoned after beating the story, can be found here:
https://bit.ly/3oXEQTy