Domino Squad headcanon:
Echo and Fives are twins, they came out of the same tube, so their batch had 6 members. Of course, a squad only needs 5, and the Kaminoans believe one twin is inherently inferior and weaker from birth due to their being twins.
So one of them was going to be decommissioned, the one who scored lower and was the less efficient. Echo and Fives, as twins, did what they could to keep in balance so neither could be seen as the weaker, but one would have gone.
Except Armlock fell into the ocean on a training run. He fell and got sick and never caught up in training or academics, always too weak. He was decommissioned, and both Echo and Fives were saved. It broke up the squad, because it was abnormal and singled them out as weaker and caused them all sorts of issues.
They were at constant risk of decommissioning but thanks to 99, they pulled back together and made it through.
In the end for too long it was only the twins that shouldn’t have made it.
Then they lost each other.
In the end Fives and Echo both lived the last parts of their lives believing they should have been the first of their batch to go, and in the end they were the last.