I feel like reincarnation could play a huge part in it. This author doesn't seem to have fallen into all of the tropes of a lot of other reincarnation stories: that is, the protagonist dies in his original world, then comes to this world with a dedication to do better and a viewpoint differing from that of the new world (usually something along the lines of: more inventive/new approaches to magic/no discrimination against demi-humans, with a huge stress on how different this new, savage world is from the peace-loving, highly-equal, highly-advanced, technological utopia-nation of Japan), with the result that he trains from birth, becoming ten times more powerful than everyone else. Then he gets a harem, because in addition to his insanely overpowered abilities, he's also gorgeous/a really nice guy.
One of the tropes of the reincarnation genre seems to be that the only person who was reincarnated (or at least remembers that they were), is the main character.
So, what I think Xiao Wu is, is the reincarnation of a spirit master from earlier times, who remembers the special relationship between man and spirit beast before men began to kill spirit beasts to get spirit rings. She went and made friends with the Titan Giant Apes, and that's why she doesn't need to kill a spirit beast to get any of her spirit rings - she's just regaining the spirit powers she had before her death.
My other theory, the one I think actually happened, is that Xiao Wu was found as a baby and raised by the Titan Giant Apes (/ or she stumbled upon them when she was very small, after getting lost in a spirit beast forest), which explains why she's such a wild child, and the reason that she does a ceremony to get a spirit ring, instead of killing something to get one, is that that is how the Titan Giant Apes gain their spirit rings.