There are key differences with Valentine's Monika than the other three girls.
-Notice that unlike the others, Monika is in a chest up view while Yuri, Sayori, and Natsuki are shown with their full bodies.
-Monika isn't wearing her school uniform like the other three but rather a more casual outfit.
-She much more interactive in the first person view. This reflects that she always faces forward and has her sights set on the player. (Not the player's avatar that the other girls are unaware of.)
-This is the only one with a non white background (Notice the sky, bushes, table, the other girls.)
-Lastly if one notices, you'll see the chocolates made by Natsuki, the gift wrapped box made by Sayori, and a letter made by Yuri. And they're all in Monika's possession.
All of this displays perfectly Monika's true goal in the game. She doesn't want the player's avatar who she found out is just code but the real player. (Anyone who picks up this game.) She became sentient and wants nothing more than to experience real life, see real people, break free of the game's software and programming.