You mean her? I'd say she has Green hair heh.
She's Airi Magama. Here's her deal.
OK in the True Tenchi Novels, we knew that Yosho had fallen in love with her during the time he was in Galactic Academy. They were together for a while, and had a baby together, and named this female child Mizuho. (She's also in GXP on Seto's ship. Mizuho I mean)
However, appearently this is not the whole story. Now that GXP has run on TV, and we've seen Airi in the flesh, and more peopel have talked about her, we also know the following to be true:
At some point, Airi must have discovered Yosho on Earth and went there, because it seems that she and Yosho had yet another child. This child was another female, and they named her Kiyone.
Kiyone is Tenchi's mother. She was pictured in flashbacks in the OVA. Tenchi calls her "Oba-chan", which was translated in the subtitles and in the dub as "grandma" - However, we now know that this woman who was pictures is in fact Tenchi's mom, Kiyone. Since Oba-chan doesn't always have to mean grandma, I imagine that was just a poet name Tenchi called her by.
So in short, that woman, Airi, is Tenchi's grandma.