I disagree she knows she is Diana because of the game, but she thinks there is nothing more on her because the character is not meant to have lore, she was just the petty villainess, if ypu have played Otome games, they are usually faceless characters , so if I got isekaied like her, I'd too think there isn't a sob story for the body I reincarnated into.
Also the protag IS KRIS thats why she tried to evade the prince and hook him up with her because originally she is his Endgame, not her, thats why she also thought that him falling for her was a "red flag" literally a bad decision in a game can guide you to a game over, the prince never gets a Game Over with Kris but he is supposed to get it if he is with Diana/the protag.
But up until now (ch 12) I found the story full of holes. Actually, more like barely thread of a story instead.
She came to a new body without the original memories, she knows shit about the life of the original owner. She doesn't even try to find out. Or what happened to the original. Or trying to pretendo to be the original.
Up until now her actions make absolutely no sense, and the only thing she cares about is pseudo romance.
I mean, it's fiction, but couldn't she think even for a second that she's in a different body and that body had a history or relationships?
She thinks of the prince loving the original protagonist and that she's in the way.
How about thinking that she stole that body and that whatever feelings sent on her direction aren't for her?