The rapport shows the relationship status with Bastian. It can range from extreme contempt to viewing the player as their greatest ally and dearest friend. The player can decrease and increase the rapport via their in-game actions. Different actions have different cooldowns, so that rapport can not be positively or negatively influenced in rapid succession.
Each companion has a unique associated non-combat perk which benefits the player while active.
Bastian has two associated personal side quests which will become available to the player once a sufficient rapport has been established. Completing the second companion quest will unlock a houseguest collectible of him, allowing Bastian to be placed and utilized similarly to other houseguest collectibles.