Some recomendations
1: 6 pics is way too few, but the limited N64 screenshots are easily augmented. There's at least two (actually four) pieces of fan art on Pixiv and one on DeviantArt and official art from both games (need to crop the logo off the AT one). Combine that with her cut-in from the GBC game's intro and the still shot from her ending (upscale with next nearest), and you can get into double digit training data rather easily (short of the ~20 recommended minimum, but simple designs like this far better with limited data).
2: All the pics in training data seem to have been given the same tag set. I'd recommend tagging the game pics "3D, low poly" (for the N64 pic) and "pixel art" (for the GBC pics). It helps a lot.
3: Tag more than just the character's standard stuff. Include the background (stadium, crowd, tennis court, tennis net), their poses (standing, holding), and objects (tennis racket, trophy)
4: Hair should be "medium hair, brown hair" ("short brown hair" isn't a tag, and the main part of her hair passes the chin, so its actually medium hair) Clothing tags are largely good, but I'd add "two-tone dress, asymmetrical clothes, asymmetrical dress, ankle socks"

I note Nina actually has a decent mass of fanart compared to Kate and is symmetrical (eligible for flip augmentation) so she should be far easier to pull off.