Taro is Tenchi's cousin. He is Noboyuki's nephew. Taro came to visit the Masaki household in Episode 8 of the OAV called Hellow Baby.

When Taro's mother became unable to care for young Taro he came to the Tenchi househod. I'm not sure who delivered Taro, but I believe that it was his grandmother. Taro's mother was sick and his father was away on business.

When Taro did come to the house he tested the girls' mothering skills to the max. By the end of the visit, Ryoko, Ayeka and Mihoshi were all completely exhausted and covered with pee. One night Taro found his way into Washu's subspace lab. At first Washu was not happy to see the baby, but when she came out to give the baby back to the girls she found them all asleep.

Washu's motherly instincts took over and she took care of him. There's nothing like a mad scientist treating you like her own child. Taro was fed healthy meals and had a lot of fun. But this time together stirred up memories deep within Washu. She revealed that she too had a child and he was taken from her all because of lineage.

Taro's father looks exactly like Tenchi, only somewhat older. I mean he really looks like him.