Well, there is definitely something very off regarding merlin's race however I do rule out her being specifically a goddess or a demon. For one thing, if she was either there would be evidence of that by now. Demon or goddess powers are easily identifiable by anyone in the manga at this point. It's not something she could realistically hide unless there is some bullshit explanation of her using her power to hide her demon/goddess presence. Another issue here is that when merlin talked about her power she talked about human lifespan.
https://jaiminisbox.com/reader/read/nanatsu_no_taizai/en/0/191/page/10
Now, I don't think she is a human either but it makes no sense that she is other stuff. She calls herself the daughter of belialin or something which going by myth is a sort of demon. So merlin having demonic origin is by no means a stretch however she is different enough from a demon herself that her actual power does not register as either demonic or anything else.
At the moment, if I had to go with a theory it would be her being a demon-goddess hybrid. Her powers lacking demonic or holy properties is the result of her not having either due to them cancelling out within her. Given her words, even her lifespan would be reduced compared to other races though her magic would be exceptionally strong.
https://jaiminisbox.com/reader/read/nanatsu_no_taizai/en/0/191/page/10
Now, I don't think she is a human either but it makes no sense that she is other stuff. She calls herself the daughter of belialin or something which going by myth is a sort of demon. So merlin having demonic origin is by no means a stretch however she is different enough from a demon herself that her actual power does not register as either demonic or anything else.
At the moment, if I had to go with a theory it would be her being a demon-goddess hybrid. Her powers lacking demonic or holy properties is the result of her not having either due to them cancelling out within her. Given her words, even her lifespan would be reduced compared to other races though her magic would be exceptionally strong.