The movement appears to be a Seiko NH71A automatic movement.
Manufacturer Seiko (aka Seiko Instruments, SII) Caliber Number NH71 (aka NH71A) Movement Type Automatic Lignes 12''' Diameter 27.4mm Casing Diameter 27mm Height 5.32mm thick Jewels 24 Vibrations Per Hour 21,600 bph (3Hz) (6 beats per second) Lift Angle 53 degrees Shock System Diashock Power...
calibercorner.com
It's a workhorse movement based on the NH35 movement Seiko uses on their low-mid-tier watches usually in the 300-500 USD range (though Seiko's been jacking up prices like mad lately). It's got hacking (the ability to stop the seconds hand) and hand-winding (the ability to wind the crown to add to the power reserve), which are standard on most movements, except on the lowest end Seiko automatic watches, which still use the outdated 7S26 movement.
The movement itself isn't that expensive by watch standards (like 50-100 USD), but it's still a significant commitment for a one-off piece like this. It's a fairly popular movement for Seiko modders, since it's an easy drop-in replacement for any NH35 watch.
The main difference between the NH35 and NH71 is that the NH7X series is skeletonized. So rather than a dial, you're meant to see the mechanical parts moving while you're wearing it, like this example:
I honestly think they're a bit tacky, but it fits with Daiya's lore and theming. I hope this guy manages to send it to her somehow.