Starting her first year in Harry Potter's third, Medea Shen showed up just in time for everything to go to hell and she's still around when things are rebuilt.

Also featuring Professor Sophronia Mombi as the potions master:

Deep, deep beneath the seas, so far down that a single mistake is death and communication with the surface is near impossible, lies the sunken base of Neo-Atlantis, a deep-water experiment.

The feasibility of undersea living is its main goal, studying the deep ocean is its second. Five scientists from around the world have been handpicked to be the first citizens.

There's also another guy who was here when they showed up.

John's character bios
Penguin's character bios