If you've not heard of Serach Bat Asher, let me introduce you to the Deathless Jewish Woman.
She was Jacob's granddaughter, the daughter of Asher. She makes her first appearance in Genesis 46:17 (The sons of Asher were Jimnah, Ishuah, Isui, Beriah, and Serach, their sister) and doesn't show up again until Numbers 26:46, in the genealogical list presented at the Second Census.
We call her the Deathless Jewish Woman (and believe me, we're doing our best not to think about that old lady from Titanic) because a Midrash, cited by Rashi, seeks to explain the reason for Serah's appearance at the census, by telling us that Jacob accidentally gave to Serach the (cue the creepy music) gift of eternal life.
The story goes something like this:
When Jacob's sons returned to Canaan with the news about Joseph being alive and also rich, they experienced an odd moment of cowardice. Not one of them had the guts to tell Jacob his favorite son was alivve. So they went to Serach, an experienced music-maker and breaker of good news, and asked her to gently tell her grandfather that his eleventh son was alive and well. She composed a song, with the chorus 'Joseph lives, Joseph lives' and went into her grandfather's tent to perform. When she got to the good part, the old man was so surprised he said, 'Ha! You should live so long.' (Though it sounds like Jacob was speaking Yiddish, I assure you he was not.)
Anyway, as it turned out, she did live for a long time, thanks to Jacob's disbelieving blessing. Or at least, that's how Rashi and the midrash explain her presence at the census in Numbers 26:46.
The Ramban has another explanation. And those of us who like our Torah with as little magic as possible, are very glad to have it.
The Ramban explains that Serach is mentioned only because she has an inheritance in Israel, ie: her descendants were entitled to land in the promised land. That's why she's mentioned at the census, along with everyone else with descendants who stood to get a family plot. Many of the people on the list were dead at the time and some of them are even woman (the daughters of Zelophehad (Num 46:33) so there's really no reason to presume, pace the midrash, that Serach had to have been living in order to make it on to the list in Numbers 46.