At an early point in development, Rosalina was a one-level character and wasn't relevant to the overall story of Galaxy, but they did have a tidbit of her being Peach's sister. However, this version of Rosalina was created by Miyamoto and also had a completely different design that was much more closely a resemblance to Peach, so she was essentially a completely different character.
The decision to completely overhaul the character and make her a major part of the game was made quite late in development, along with including Lumas (before Lumas, they had "Cosmic Toads" which were Toads with Stars on their head instead of polka-dots, indicating they were not residents of the Mushroom Kingdom). However, the current, final version of Rosalina was not created by Miyamoto, but by Yoshiaki Koizumi (current producer of the Mario franchise, and was a director on Super Mario Galaxy. However, prior to Galaxy, his most notable development role was working on Zelda games, most known for being the scenario writer for Majora's Mask. That's why Rosalina as a character and her backstory feels more like something from that game than a Mario game). This decision did result in certain things being scrapped. Starship Mario was originally going to be the hub of Galaxy 1, but then they made Rosalina's Observatory instead since it made more sense at this point to give her a place to call home. Also, certain cutscenes in Galaxy, were originally just part of Rosalina's storybook, but were made into full-on cutscenes, apparently.
I was talking about Ludwig, and more the specific KINDS of fanart I've seen involving the two.
I actually ship Rosalina with Bowser myself. They're my OTP.