[xris]

The ending is left open for your own interpretation but maybe it helps if you read the last chapter again (and look at the actual pictures as well as the words).

Spoiler for Manga ending:

The last chapter shows that Seiji was about to burn the diary that Midori wrote (while she was his right hand) but he was interrupted by the "real" Midori confessing her love. A month later he buys her a new diary and the final image we see is that of the new diary beside the small diary with the words "Hey, Midori...", "Yes, Seiji?".

I take this to mean that Seiji didn't burn the small diary but gave it to Midori after he gave her the new diary. This would mean that she eventually learns of her being Seiji's right hand. I would assume that whoever wrote the comments in Wiki came to the same conclusion but clearly this viewpoint is only implied.

Edit: And of course there is a 4-koma at the end of the last volume that does actually state she read the diary so I guess it isn't down to any interpretation anymore