Well, we have Alessa directly telling her origin in "Gifts" so... I mean, feel free to toss it in there.
Similarly, the Second portion of "Legacies" has Raef's, but that would be problematic.
Loss does a job of Zhiran showing his origin to Bartholt.
For my characters, we have: Advent of a Planeswalker (Mari), The Planeswalker's Daughter (Ellia), Before The Dawn/The World At Dawn (Larasa and Morgan, take your pick of story), Homecoming or Corrosion for Illarion, Night Voices (Amah), Morgan's Notes: Quiet Lia (Lia Xin)... Yeesh, I have a lot of characters.
Hmm... I think the Eightfold Lotus might cover both Kimberley and Jinsen.
I guess I have a couple stories that could fit this theme. "Small Magic" is kind of Beryl's origin story, although it's "A Bet on Kindness" where she becomes a 'walker.
"The Wind and the Waves" ticks both boxes for Gale.
If we're including planar natives, then "Foxtrot" could work for Jackie and Trotter.
Suggested Stories Therefore
1) Gifts (Alessa)
2) Legacies (Raef)
3) Loss (Zhiran)
4) Advent of a Planeswalker (Mari)
5) Before The Dawn/The World at Dawn (Larasa and Morgan)
6) Homecoming (Illarion)
7) Night Voices (Amah)
8) Morgan's Notes: Quiet Lia (Lia Xin)
9) The Planeswalker's Daughter (The Endbringer)
10) The Eightfold Lotus (Kimberly and Jinsen)
11) Small Magic/A Bet On Kindness (Beryl)
12) The Wind and the Waves (Gale)
13) Foxtrot (Jackie and Trotter)