Hmmm 🤔
Crazy theory that's definitely not gonna be the truth:
On chapter 1, page 17 we fan see in the TV something that resembles a head of a typical alien: big eyes and a large and pointy head. The news seem to talk about something that happened at night. Could this mean there was some sightseeings of an alien around 10 PM? Was this alien murdering something, someone? Now we see Seiko in a shock mode, or is it?
Vsauce music starts playing
A lot of representations of aliens around the world seem have characteristics named before: big eyes and big head. When you're heading to the store and then see a person with a massively elongated head and two big large pitch-black eyes, you think: "Oh, that's not human". Aliens are easily recognizable that way, isn't it. But what if we can't recognize them? Dr. Helen Sharman, the first British astronaut in soace, said in an interview: "There are so many billions of stars out there in the universe that there must be all sorts of different forms of life. (...) It's possible they're here right now and we simply can't see them."
Now, going back to the manga, is it possible that Seiko is in reality an alien? She does behave suspiciously in these 7 chapters so far, now we see hee mumbling some words with absolute nonsense. Are those phrases the default ones aliens use to infiltrate and disguise themselves amongst humans? Is it possible our MC is the chosen one by aliens to save humankind or to prevent some sort of apocalyptic disaster involving the whole galaxy? Who knows. Probably Seiko's just an alien who want to have a happy family after watching how humans behave for a long time. Only reading the next chapters will give us the answer.
TL;DR: Theory: Seiko's an alien lol