I'm not sure if there's anything else, but Agnes is the primary connection to the canon ending.
Basically you have to follow Agnes' plans and such. If that doesn't work, then I'd guess there's at least a minimum affection number you need for her. I usually go all out and try to get and do everything, so I didn't have any issues with any of the three ending I got. Another spoiler, there's one with Azumi and one with, let's say The Director, to avoid big spoilers for anyone else.