itch.io

Firstly, if you hover your mouse over an item (or click on an item to see its details) and it says it is a "gift," then you can use it to increase affection. Though I do not know if there are any variations of the gifts yet, the main one to buy is just called "Trinkets"; the icon for it looks like a gold necklace with some rings around it. It costs 2500 gold to buy, and it gives 25 relation.

I don't know if this is secretly a feature or something that I don't know of, but I thought it would be cool and add more strategy to the game if the Species you unlock give more than just a physical change to the invaders. I think it would be cool if, at the very least, different species are more likely to get certain traits than others, like the demons being more likely to have the "scary" trait or something like that. Or at most they could have entirely different things they are good at, like the elves do more damage with magic attacks, the catkin are more likely to succeed when being sent out on expeditions, or the slimes have a higher chance to dodge attacks against them.

You can also buy gifts that sometimes appear in the auction to gift to followers that improve their relationship with you a bit. You can also see their relationship level with you by looking at their details just below Hygiene.