After Rinwell joins your party and you leave the village of Messia 224 in Cyslodia, you can have deeper conversations with your party members as you rest around campfires. After you have decided whether you want to cook something or not, you will get a corresponding menu with all party members.
If you see a speech bubble with three dots behind their names, such a conversation is available and you can deepen the bond with the respective character. If you don't see a speech bubble, you can skip this menu and select "Sleep" directly.
After about 6 conversations with each character, you will see a smiley face behind the characters, which characterizes the maximum bond. You will also unlock the ultimate Arte and a separate trophy for the respective character.
Unfortunately, you can't have these conversations one after the other. In between, there is always a cooldown of about 30 minuts to 1 hour in real time. So you should never skip these conversations from the beginning, otherwise you may have to wait a long time in the endgame until you get this achievement. If you have reached the maximum bond with all 5 party members (Alphen is not one of them here), you unlock the success.