Hi Krblshna, if you take a look at the example projects that come in the Spine installation folder, you'll notice that the most common setup pose they have is an idle position.
For the way Spine works, every animation is relative to the setup pose, so if you move a bone in setup pose, it will reflect also in all of your animations. This is great when you need to add new gear, or slightly change the posture of your character, etc. but as Puppy said, it's something that requires a bit of getting used to. A good habit you need to start having is, as Puppy suggested, to test your rig and poses in animate mode, and make the needed adjustments in Setup mode.
Regarding multi-pose character, you might find this series useful: Spine Twitch Streams - YouTube
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It revolves around reusing the same bones to attach different angles of the same body parts. An addition to that is that you can also place your images in setup pose, turn them into meshes, and then update the bindings during animate mode to get the new pose correctly.
You can see an example of this workflow around 2:47:40 of this video:
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