As a WMG subpage, all spoilers are unmarked as per policy. You Have Been Warned.
- Specifically, the person in the prologue, the one who magically explodes into a bunch of colourful orbs. These orbs are seen to scatter in various directions, one of them enacting a meteor crash in front of the astonished mrs. Jacobs. This would explain why "fifteen years" recurrs so much:
- If Arianna is the little girl snooping in the prologue (she would have been seven years old if it's indeed fifteen years prior to the plot), the explosion would plausibly make Zera think she must have died. She might have really been killed, then revived by the magical orb.
- Frankie is fifteen i.e. was activated (became sentient?) fifteen years ago. Possibly he was just a regular robot before then.
- And Vector is also fifteen, Happily Adopted by Mrs. Jacobs who later tearfully admits to having nobody else - she specifically says she lost her husband, but doesn't say when. Perhaps the magical orb latched itself to her unfulfilled dream of being a mother, creating a Wonder Child?
- Marina and Rori are over fifteen, but only a little - they may have been hit by the magical explosion shrapnel as a toddler and a baby, respectively.
What's more, tracking spells don't work on them - Zera's spell for Arianna fails, and so do Arianna's spells for the other two and for Rori. Notably, Arianna doesn't interpret this as the boys having died (like Zera does), she just tracks someone else she thinks would be with them. So, since the boys are actually not dead, and neither is Arianna, tracking spell failing doesn't necessarily mean the tracked person is dead (and assuming so would be a Converse Error). It might be the most common cause, but not the only possible one. And if they are all the same person, then the tracking spell would be confused when one part of them is trying to track another (like tracking your own leg, only it appears to be on a different continent, and your kidney on yet another, and so on... confusing).
Later on we learn
Arianna read that spell in the book from mrs. Jacobs shop and she thinks the spell is faulty. Obviously, Zera must have learned it elsewhere - maybe his training said failure to track means death.
As of this page we now tracking spells work by locking onto the trackee's aura, which is unique to a person. Lack of aura would make one untrackable. Arianna thinks this is the case with Frankie but is puzzled as to why Vector or Rori would have none, and of course never has tried to track herself. This supports the "parts of a person" theory.
- Possibly, they are (were, before growing into people) the explodey person's emotions:
- Arianna, the Red Is Violent Sir Swears-a-Lot is their anger,
- Vector, the Supernatural Fear Inducer is their fear,
- Frankie, the Genki Guy is their joy,
- Marina, the motherly Healing Hands lady is their compassion,
- Rori, The Gadfly with Super-Speed is their excitement.
- Sage, the guy with a Mad Scientist approach to magic, would be curiosity in this arrangement.
- Arianna's brother. He died fifteen years ago, as far as she knows, executed for high treason she doesn't believe he would commit. But the medical report Zera's lieutenants found says something different (assuming it's not about Zera's sister, but Zera himself, who may or may not be Arianna's brother, or another fragment of Arianna's brother). Can we be sure her memories are accurate?
- Come to think about it, maybe the explodey person exploded because they were artificially given magic? And the titular "castoff" would be the magic not absorbed by their body, which went rouge, so to speak, and hit other people.
- Possibly it was even Sage - powerful magic, he appears in Arianna's flashback shards, has scars on his hands, so has been through something dangerous, maybe an explosion.
- Sage's scars (part of them, anyway), are caused by him experimenting on himself, but still. If anyone would volunteer for this sort of a thing, it's him.
- It's not impossible that the explodey person had been Nico (Sage's friend), either.
- And a product of a Maligned Mixed Marriage, given the Fantastic Racism we've seen towards elves.
- This appears to be Jossed, since Zera all but said he was adopted into a human family.
- ... but the person he's been framed for murdering. His "high treason" is supposed to have been assasination (one more reason for Arianna to not believe it). Maybe someone was killed, just not by Arianna's brother? And he got framed for it, then (as a convict) used for the dangerous magical experiment?
