Pokey Giygas's host

Ness offered his arm, and Buzz Buzz settled down on it, folding up his wings.

<If I had been able to destroy the egg, it wouldn't have come
to this, but unfortunately...I hadn't properly calculated any of its coordinates prior to
these right here. And so...Giygas has been unleashed upon this world. And on top of all
that, we don't even know what Giygas' larval state will look like. The newly-hatched
Giygas will have assumed the form of your typical, average, every-day human being.
He may have taken anyone who happened to be nearby as a host.>

“A...host?” said Ness, as his heart began beating even faster. “You mean this
cosmic monster could have possessed someone?”

<Exactly!> cried Buzz Buzz. <The monster will grow inside the belly of an
ordinary person. And a human being carrying Giygas would probably develop a
wicked, treacherous, incredibly detestable personality. This is because Giygas would
be controlling them from within. These waves of evil influence would affect more than
just that person, they would spread out into the surrounding environment...and if that
person had an even slightly twisted personality from the start, they may begin to
sympathize with Giygas and his hostility against the earth, align themselves with
Giygas, and become Giygas' puppet. ...And then, over the course of about half a year,
slowly and surely he will grow. After the preparations for his final transformation
have been completed, the monster Giygas will begin to eat through his host, until
finally, he will emerge. Once it gets to that point, there will be nothing that anyone can
do. And so, before that time comes, we must seek out, identify, and then eliminate the
one who has become Giygas' host. ...And in this whole world, there is only one person
who will do it, only one person who can do it. A young boy named Ness!>

- Mother 2, Chapter 1 pg. 13