The thing about Ken, is that it is a concept that developed over time, and thus has the problem of essentially being a rough draft of a character, with bits and pieces of various things, from where origin and story were changed several times to make a better and more cohesive concept.

But the 'dark ritual' involved is essentially very similar to ones from Japanese Folk-lore. IF a great injustice is the source of the death of a creature, or is visited on the spirits of the world, it becomes a 'Hengeyokai', or 'Changed Spirit' as Scrib described it. This is in contrast to just a 'Yokai' as the 'Henge' part implies it has changed for the far worse. Where a 'Cat' or 'Yokai of a Cat' are not nessecarily evil, if you were to kill it in a especially horrible way, or desecrate the remains in a profoundly disturbing way, it comes back vengeful to the point of essentially being evil. It also usually implies a explicit (Albeit unwanted) increase in power, compared to what it had in life, or even powers it never had before.

That last bit is especially useful in the particular setting where 'Evil Aligned Ninja and the Magic Users who support them' are inclined to binding evil spirits and demons to do their bidding, or even enslaving their essence directly into their body to amplify their own power. And thus 'Ken' ends up being a purposefully produced creature rather then an accident of man's malice, the power of hatred for her new masters twisted as her mind was pounded into a state conforming to their ends, to the point of becoming a stoic vessel of rage and violence. A steely and stoic exterior that watches and waits, biding her time and bottling a torrent of various violent emotions as per her forced discipline, until it is time for it to be unleashed in full.