unexpected shade comes out

Hi I’m newbie to Blender

I brought this charactor from Koikatsu

and I put it in Blender and clicked ‘Non Manifold’

as you see, it shows that vertex and edge are flawed, which means they are seperated.

So I merged them

but as you can see, unexpected shadow appeared like that.

I don’t know why

Why this happens? and how can I sort it out?

Hard to say by just looking at an image but I suspect there are internal faces, possibly a mouth interior which you could either separate or re-model the part where it joins the lips.

I see.
but the best solution is to re-model? really?
Is there any function to flip over those faces in Blender by one-click?

I’d say the safest way is to separate those inside faces.

I would make a copy and try the data transfer or weighted normal modifier after the operation. There’s also an addon called y.a.v.n.e. (yet another vertex normal editor).