More or less, I'm just talking about sprites. The game core, as it is now, supports 8x8 sprites. If I were to, say, make Rika's sprite 16x16 pixels and be highly detailed, I would probably have to do a hefty bit of rewriting of the game's core. And that would take forever because I suck at sprites larger than 8x8 pixels.
Metroid: Return to Maridia (dead) and
Mahou Shoujo Lyrical Nanoha: Battle Lyrical (on hiatus) are prime examples of my inability to code for larger-than-8x8 sprites
So, Rika will probably have a similar or identical sprite to Nanami, but other than that, there will be plenty of graphical differences
I can reveal one such difference now. Unlike in Yumé where the game's sole dialogue came in the form of Nanami's diary entries, Yumé 2 has text-box dialogue.
Mainly, the reason for this is that while Nanami is more shy and keeps to herself, Rika is more upfront and talkative. There are plot-centric reasons for this, too