The window in the first panel isn't open.
The window in the end is.
Nobody is shown opening the window.
I take this as memories of someone who loved someone who left via window.
And can't forget they used the window in their special place.
And sees them sometimes when they visit that place.
Her angel -- the part of that person -- lives on only in her memories now, and she can't let her go.
Maybe I'm wrong. I often am.