This is done by completing the corresponding Character Story (marked on your map by a blue indicator) and fulfilling requests from your party members. Maxing out your intimacy with another character is the first big requirement towards partnership you'll also need to complete the main story.
Once you have more than 50 followers and meet other requirements, you can become an affiliate and earn cash from subscriptions, Twitch bits, and brand deals. As a partner, you can boost your earnings even more with ad revenue.
Affiliate marketing is a process where publishers earn a commission by promoting a product or service made by another retailer or advertiser using an affiliate link. The affiliate partner is rewarded a payout for providing a specific result to the retailer or advertiser.