Boy, Elisabeth really leaned into the whole dark wizard schtick, didn't she? Made for a simple, straightforward mission for Mira's first, at least. The gag where the elder agents couldn't explain everything out loud to avoid spoiling Mira was a unique idea. I also liked taking advantage of the third partner to play different card games, and the detail about collecting chocolate frog cards.
The assassination got a little dark, there, with Mira's actions. I hope that's addressed in future missions? We don't want actually evil agents, after all. Also, I'm not sure the squid would actually accept a body like that, since it protects students in canon (rescuing Neville)?
There's a missing paragraph break between ". . . pulled an item about the size of a finger from her pocket and put it down." and "Edward was attacked by a fit of curiosity . . ."
There's an extra paragraph break in the middle of the sentence, "'. . . but I know that’ll set you up for the inevitable pun so I’m not going to do it.'"
And in the charge list, "'. . . being the uncanonical nephew of Sirius Black . . .'"
Aelin had female pronouns in the fic, so this should be niece.
Also, I feel like you should have some kind of clear scene change indicator before "The Sue had apparently been annoying her godfather with stupid puns for the entire car journey." As it is, it seems like the agents are just abruptly walking from Elisabeth's house straight up the steps to King's Cross, and it's jarringly sudden.
Also also, I'm confused by the following paragraph:
Kat grinned and closed the portal, then opened another one to what looked like the Slytherin first-year girls’ dormitory. “Just something I need to do before we go,” she said, and stepped through the new portal to what looked like the Gryffindor first-year girls’ dormitory.
It reads like only one portal has been opened, but two different dorm rooms are visible at once? Halp?
—doctorlit, seeing too much of girls' dormitories, but in a spacial inconsistency way