Well, in that case, let's go whole hog. I'm assigning the memory of Diamond's transformation to the earrings that look suspiciously like Diamond's earrings (and therefore I've got an argument for canon recurrence), and I'd like to assign Raquel's sentience to the Vermini encounter (as there's no way I'd ever find a recurrence for that).
...The problem with that is, Raquel comes from Rikka's timeline. So the question is, is it the "same Raquel" that knew Rikka, and he'd presumably have to Echo back his memories of his original mistress, or is it a "new Raquel" that hasn't met Rikka and hasn't had a partner before Winter? I'm not sure which is better here; I'm partial to the second to keep things slightly less insane, but I'm willing to work with either. So:
Reason for Echo: Sometime after the first one, Winter runs into a Vermini-infected creature. What is returned: Raquel's sentience. I'm not actually restoring any of his movement modes yet, or any of his other powers; just the ability to talk and think, and have a functional face and ears.
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...The problem with that is, Raquel comes from Rikka's timeline. So the question is, is it the "same Raquel" that knew Rikka, and he'd presumably have to Echo back his memories of his original mistress, or is it a "new Raquel" that hasn't met Rikka and hasn't had a partner before Winter? I'm not sure which is better here; I'm partial to the second to keep things slightly less insane, but I'm willing to work with either. So:
Reason for Echo: Sometime after the first one, Winter runs into a Vermini-infected creature.
What is returned: Raquel's sentience. I'm not actually restoring any of his movement modes yet, or any of his other powers; just the ability to talk and think, and have a functional face and ears.