Missing-Nin Rank is perhaps the most unique Rank available, since they can wander freely across the world. Because of this, it is a naturally popular rank so to prevent overflowing of these Lone Wanderers, users may NOT start with Missing-Nin Rank. Instead, to become a Missing-Nin you must abandon your Village in-character.
When abandoning your village, each post you make must contain AT LEAST 500 words. Also, it is impossible to actualy finish your leaving topic until a full 5 days have passed in real life. Within these 5 days, anyone from the Village may choose to attack you, and it is possible for one of your pursuers to inform the Kage of the Village, should you manage to escape and successfully become a Missing-Nin. If this happens, a bounty can be put on your head. With a bounty on your head, people will be able to gain money by killing you or bringing you to your village alive, leaving you at the mercy of the Kage. Also note, that if after 5 days you are currently being attacked, then the time until you can end the topic is increased by an extra 2 days. If you kill your pursuers before those 2 days are up, then the topic is automatically ended with you successfully leaving the Village. If however after those 2 days you still have living pursuers after you, thenthe topic is automatically ended with you successfully escaping.
What this means, is that you can choose to kill off all your pursuers within the 5 to 7 day time period, OR you can simply stand your ground and survive the onslaught for 7 real life days. Either way leads you to successfully gaining Missing-Nin rank. As a Missing-Nin, you are forbidden from joining the Chunin or Jounin Exams. For this reason, you are simply marked as being the Rank you were when you left your Village. The only way to progress, is to kill a Shinobi that is 1 or more Ranks higher than you. If you do this, then you will automaticaly be promoted 1 Rank. Take note however, that no matter what Rank you are compared to the other Ninja, you can ONLY EVER be promoted a single rank meaning you can NOT skip Ranks.
After becoming a Missing-Nin, unless there is a bounty on your head people have no real reason to kill you, except for the people of your former village since you are not yet a criminal. If as a Missing-Nin you perform a crime that is witnessed (be it by another user or an NPC), such as theft, Murder, assault or armed assault, or any other clearly-illegal activity, only THEN are you considered a criminal and are marked in the Bingo Book. If however you perform a crime that is not witnessed, then you get away scot-free...... hooray for you!
The Bingo Book is a nasty piece of work, mainly because it marks your name, appearance, Elemental Affinities and your highest ranking Jutsu(s). When marked in the bingo book, you are also given a "Criminal Level". This is based on how many crimes you have commited and the seriousness of these crimes. The levels are as follow:
Petty: These are small-time, unimportant criminals who are worth little to nothing when arrested or killed.
Notable: These are still not worth the hassle to kill for their somewhat low price, but it IS higher than the price of a petty criminal, and they are also more dangerous.
Dangerous: These are hardened criminals who have decent prices on their heads. Hunting them may be a good way to rake in some Ryu, but they are also somewhat dangerous in combat and are not to be underestimated.
Severely Hazardous: These are some of the best Criminals in the business, and as such are extremely dangerous in combat, but are also worth huge amounts of money. Opposing them is a huge risk that will probably lead to your death, however if the risk pays off, it pays VERY well.
Most Wanted List: These are Criminals that could single handedly bring a nation to its knees, with only medium effort. They are, more often than not, the heads of powerful criminal oganizations, and are always of Kage level in power.