I did some experiments with memory hacking on the Japanese DS version last night. I didn't find the memory address for the active party at first, so my first experiments were done by editing the loaded location events to add the 8th character to the party. I've attached an image of what it looks like without any other adjustments.
- Name is Epoch
- Default stats are zeros
- Sprite in menus is Crono's menu portrait
- Save portrait and menu icon are palette-swaps of Crono
- Blank, black full portrait in menu
Hacking a LoadPC command to load the 8th character produced a black screen.
For reference, memory addresses in no$gba's debugger:
~207293B: Character stats, starting with Crono. 0x60 bytes each, possible differences in layout to SNES.
21C2000 Location event data
21D0980: Active party