There Are Various Ways of Defining the Devil

a child of many names
The Devil is also called for example Lucifer, Satan and Beelzebub, just to name a few.

Whatever Penalizes Certainty of that Care is Worth it for All
It might be that it is when (and to the extent that) one looses that type of certainty that one looses the soul.

The Concept has Developed through History
Nowadays, the concept of the Devil is very very well-known. ... In very many conflicts, it can be used about those on the other side (or sides) of the conflict  -  not too rarely by representatives of both (or all, if there are more than two) sides. It has had and still has a lot of potential influence on people opinions.

My Guesses about the Ancient Hebrew Definition
The ancient Hebrews aspired to understand what evil is about. They had early onset of some deep such understanding. However, eventually some of them developed aspirations to do so, which were shallow, I think  -  and these were perhaps also the ones that brought about turning their beliefs into religion. The new aspirations can have meant being into pretending that wisdom is to pretend to target evil when one targets ability to present oneself as if evil. I believe that these were also fairly easy targets for evils that did not seem as bad as they were  -  and that this can have been noticed by those of them that were not too far into such shallowness. In order to warn those who were at risk for it, these (who noticed) formulated the devil concept in order to define and warn about such evils!

Supposedly 666
The number 666 is actually 37 * 2 * 3^2, which is hardly an interesting enough number to be called something as important as Satan. Thereby this Satan's is almost not applicable for what the bible and so says that Satan is. But, if one interprets it as as three sixes in another sense that number of tens to different powers, then there is a way to make sense of it. Because Satan is there in twenty-nine,and twenty-nine ingratiates six, which can be interpreted as that he supports what six is about. That the number of sixes is three can be interpreted as a way of saying that it is six through two ingratiation steps  -  the first one being eleven, which could then be called 66. Thereby 666 is through math a fairly applicable definition of the Devil.

So in What Sense Is 29 a Huge enough Evil in the Sense to Represent Satan?
It is because the essence of reciprocity rests in three that six is the essence of interrupted and thus corrupt reciprocity. Because in two there is the essence of interrupting oneness from being infinite and eternal. So a two with a three corrupts the oneness n three, i.e. of reciprocity, interrupting it from being there for all. Moreover, being the second ingratiation step of such impurity, twenty-nine ingratiates such evil, and thus forms an extremely dangerous symbiosis between corrupting corrupting oneness of three especially and oneness generally

What Are the Scenarios When Lucifer Becomes Satan?
It is when a twenty-nine also posses a forty-one (which stands for essence of meaning in curative tolerance in circumstances of corruptive oneness interruption) that it becomes the totally extreme evil that manipulates so that an angel who can induce evil for goodness sake -  and does that more or less exclusively  -  starts to induce evil more or less exclusively for evil's sake. That is when the seven (tolerance), which Lucifer in his unfallen state connects with a 29, no longer can use it for tolerance and worthwhile attitudes about considering evil to be a scourge to be dealt with as food for thought, as an eye-opener and a lesson.

Somone Who Eternalizes Problems

... and that causes very much more problems than most other creatures that are at all established as God's allies. I say at all because there is a genius of good in caring about that the devil insults those who are evil enough to ignore that one should care for all that one can. That is he hinders the essence of trying to have it that everything is excused by some kind of end goal.

He does that by establishing contact between evil and good that justifies making the intimidate punishments long lasting. It is not the medial contexts that are totally established for the sake of the beneficiaries of the benefits of the crimes that happen to the inadequates for the given situation of a war crime or so. Instead it is about those who survive those catastrophes  where one has to commit crimes against justice in order to establish justice back from some tyrant, such as for example Stalin, Idi Amin or Hitler.

It is they whom the devil starts to pester until their karma is proven strong enough to withstand his pestering. They cannot establish their contacts well enough to resist his (the devil's that is!) punishments unless they try very hard to pertain to everything God says (so to speak) that they should pertain to. This does absolutely not mean that they must pertain to some written or spoken words. It is not the so-called holy Bible or the Koran that establishes at all the wishes of God. Nor dies the Talmud do that, although it causes less misery for the not so guilty people, whereas the two others have caused total establishments of evil-as-if-good situations. Similar holds for other holy books.

When one is punished slightly, then the devil threatens one's soul's survival until one has clearly expiated for whatever one has done. It is not small that this sort of Godly creature actually threatens with making short term penalties eternal. Thereby, those who join him in trying to do that, are usually in the end penalized for it, in ways that usually pertain to eternal damnation for real. This some of them try to hold against me and other people who have in previous reincarnations done good with harsh methods, and were unfortunate enough to fall into something of a devils trap about that.