Pillars of Eternity II: Deadfire

In the Reputations, menu you can check likes and dislikes of all the companions currently in your party. You can also check their attitude towards you (and each other). If you express opinions that a companion approves of, you will gain reputation with him. Roughly counting, you need about 8-10 such reputation-increasing events to maximize a positive relationship (to get it to +2).
I found it the easiest with Edér and Xoti – they like clever dialogue options (which suited my character just fine) and as such I got them to +2 in no time.