Favor and Memory decay.
After raising NPC intimacy for keywords yesterday, I went back to check how they felt about me. Using Stewart as an example: I successfully unlocked his secret shop yesterday while digging for Keyword responses. I used mostly Likeability Potions. 24 hours later, he is still familiar with me (Memory), and gave me the "Great Interest" (Favor) cue. He also initially gave me the "Stop Conversation" cue, likely due to (Stress) being raised from interaction. His secret shop is not accessible currently.
This means Favor, at the very least, is decaying but very slowly.
I wonder if, similar to Partners there is a "base value", and every hour the value of Favor moves towards that base. For Partners, it is 120. For NPCs, perhaps it is 0? I assume this because the value can be in the negatives for NPCs, as some NPCs with Negative Favor have unique portraits.
Additionally, Partner Stress decreases by 1 every 10 seconds, is very fast comparitively. It may also be a similar case for NPC Stress.
I have no idea how Memory decays, but I had once read that it decays slower once it has been raised over a certain threshold, but I have no way to prove this. I want to examine if maintenance causes NPCs intimacy to hard reset.
Also, I think a visual representation of how Favor affects certain NPC portraits would be a good addition to the page. Not all NPCs do, but many have 5 Intimacy-connected portraits; Hate, Bad, Normal, Good, and Love. (They can be seen in MabiNovel.)
EDIT: I ended up adding the visual representation myself just to see how it looked.