I truly think that Casey did not realize her feelings for Derek the entirety of Life with Derek. I think Truman's Last Chance is when she finally put the idea behind her that Derek is an undatable, skirt-chasing cad, but I don't think she made a connection to herself. Even in Futuritis, when she prompts him with "Same Difference" sure, there's something deep within her that wanted to know if he saw her as a sister after all this time, but she doesn't analyze it any further. And of course, Derek takes that as "she sees me as a brother. Great." And pushes all of his feelings for her down.
She never confronts her feelings for Derek within the Life with Derek timeline and it's so devastating. Like Derek has known from the bat that he was so down bad for her and his feelings only grew stronger with time. I'm in PHYSICAL PAIN.
Can we talk about Life with Derek Season 4 Episode 8.
When Casey is talking to Paul about Truman. Paul clearly is under the impression that she is talking about Derek. Even the way he says “Oh, we’re talking about Truman” with his shoulder shrugs. He’s like this is new information wasn’t expecting this name to be said.