"portal" effect?

Look at the above screenshot.

Im not aiming for effect like in the Portal game (the portal doesnt show another part of the world, its just textured). I just want a part of my character which is on the other side of a quad to not be rendered.
I could change the rendere order, but then it would act the same way from the front side and thats not what I want.

It’s called a clip plane. Just create one and put it in the scene.

I didnt know you could make one yourself.
One last question, what if its not a plane but a character itself? Like a superhero absorbing bullets or a cartoon character swallowing a very long burito? Can the clipplane have a different shape?

A clip plane has the shape of a plane. If you want to use anything else, you’ll have to use multiple planes, a shader, or something else.

I’m a Panda3D beginner, but the theoretical idea I’d be thinking in terms of would be a combination of Z-buffer and a clip plane stuck somewhere in the middle of the character.

I’m not familiar enough with Panda3D to know how to implement that idea.

One common way to clip out exotic shapes is to use the stencil buffer. It does take a bit of practice to use the stencil buffer effectively, but this is commonly done in many graphics engines and there are plenty of examples on the internet.