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