Affinity is a hidden value in the game that is affected by Private Actions and your responses to certain questions by party members during a private action.
There are also two items that directly affect the affinity of a character: Love Potion No. 256 and Emotional Destabilizer. The former increases the affinity of the character you used it on towards Fidel. It also requires an incredibly rare material called Lezard’s Flask but I recommend just focusing other medicine recipes through Synthesis since there’s a large chance that you’ll get this potion as a by-product from 40+ impurity synthesis attempts.
Emotional Destabilizer can also forcibly reduce the affinity of a character towards Fidel. Be careful when using it though since it will also affect the ending. If you need to craft more Love Potions, the Ruddle Shop in Central Resulia should have the ingredients to synthesize them but it can be pricey (Ruby x1, Iron x1, Blue Roses x4 = 20/43 quality/impurity ratio) Love Potions and Emotional Destabilizers seem to appear frequently between the 42-43 impurity range. Of course, always save your game before mass synthesizing so you can reload in case the batch doesn’t produce desirable results.
Also, due to the multiple possible endings in the game which will depend on the characters’ affinity to Fidel, it’s recommended to craft 20 E. Destabilizers and 20 Love Potions while you still have access to the Ruddle Shop. That way, you can manipulate the endings by forcibly lowering/increasing the affection level between the characters before the final boss fight.
Ending Clue *spoiler!!*
Spoiler: