Multiple realities
(covers information from several alternate timelines)
Yoshiko was the daughter of Hoshi Sato in an alternate timeline in which Enterprise NX-01 was sent back 117 years into the past. She had a brother named Toru.
In the normal timeline, Hoshi learned of Yoshiko by searching the database of the alternate Enterprise, but refused to learn who the father was, believing that "some things are better left a mystery". (ENT: "E²")
This character was only mentioned in dialogue.