CFrames can be used to achieve this. I recommend reading up on CFrames since they’re quite helpful. In this case, you can use the following code below to position the sphere behind the character no matter the direction they’re facing. Of course, you’d want to update the values continuously when you implement it into your game.
-- followPart is the sphere; character is the character model
followPart.CFrame = character:GetPivot() * CFrame.new(0, 0, -5)