Dormer Wall Type @ Elevation Change?

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I have created a room in a second story attic space so to speak. The room juts out of the main roofline like a dormer. I need to change the wall type and follow the angle of the main roofline so that the interior materials match that of the dormer room and stairwell, and the exterior material remains in keeping with the remainder of the exterior of the house.

See the pictures below for detail.

Basically I need a wall made of 2 materials, the bottom half of drywall on both sides, and the top half of drywall on the interior surfaces and siding on the exterior surfaces. This wall also needs to change at an angle following the 8-12 angle pitch of the roofline.

Any suggestions?

Thanks