Everyone really most dont go to deep into story from my limited experience. That said there is so much to this game maybe I'm just not triggering a flag to advance. Robin would be my guess though. Pretty much begins and ends with taking up Robins debt and that character feels best to "a way for non-temple characters to lower world corruption routinely " as the dev said hes working on.
Considering that it's both 'neglected LI' and 'non-temple way to lower world corruption', my guess is that it would indeed be about Robin, and the 'non-temple way to lower world corruption' would probably involve the Orphanage. As you say, Robin's 'content' is pretty much whether you take on their debt or not. I don't really consider being able to hang out in their room and their lemonade/chocolate stands as 'real LI content'. Definitely not Morgan, considering that they're not even a proper Love Interest, and as for Avery, while you only really meet them every Saturday, already has enough content for their purpose (to me, anyway - I only see Avery as a walking wallet, but often I end up being annoyed from their persistence on Saturday that pretty much turns him into Kylar with a car).
Though well, the 'neglected LI' it could very well be Avery, if the next update would include being able to visit their mansion instead of needing to wait for them to find you on Saturday. But considering I feel like that should be a Stockholm Syndrome of a 'passive' kind, where it progresses the more you go on dates with Avery where you take the money afterwards, especially if it was after sex or the hotel 'business purposes' date. Kind of like the more the PC do it, the more the PC becomes 'addicted' to taking money from them, where eventually the ability to refuse becomes harder and harder (Willpower check eventually becomes required and becomes increasingly difficult the more you take the money), and the PC ends up somehow 'signing' themselves into Avery's ownership or something.
As for 'non-temple way to lower World Corruption', I certainly hope it involves the Orphanage as mentioned. As it is, what happens to the other Orphans doesn't really matter at all right now. The whole Hope/Rebellious thing exists only for some very minor flavour events and for the purpose of getting the Silver Brooch for the Museum. Overall, even as World Corruption rises and the town starts going super rapetown mode, none of the Orphans get involved in the 'raping' part, but rather are the victims (mostly as a side effect of needing to pay Bailey, as usual). While it's not the same as the Temple, something about 'faith is stronger than (?)' - an Orphanage with a strong hope for a promising future to me makes sense in helping reduce World Corruption as the Orphans end up not falling into depravity, creating a sort of collective mental barrier that makes it harder for the Cult to touch them.