Gotei 24

The Gotei 24 24 Division Imperial Guards, or 24 Court Guard Squads in the English dub) is the organization which most Shinigami join, and one of the three main branches of military in Soul Society. It is split into twenty four divisions (hence the name), with several divisions having certain specializations. The Gotei 13 is subservient to the edicts of the Central 46 Chambers. They also have no jurisdiction over any of the noble families. The symbol of the Gotei 24 is a rhombus, all the 24 Divisions symbols are placed within the rhombus on the back of the Captains haori. The Kido Corps, not officially considered as part of the Gotei 24, is still under the authority of the Central 46, and works together with Soul Society, it is why it is actually here, but officially, both Kido Corps and Omnitsukido are not part of the Gotei 24.

There are a list of fan-made Shinigami characters who are a captain, lieutenant, or seated officer and also most of canon Shinigami.

Positions
A Captain (隊長, Taichō; lit. Unit Commander) is the leader of one of the thirteen divisions (with the special case of the Commander-General). A single Gotei 24 "unit" (隊) is estimated at about 200-500 troops.

Captains are generally the most respected shinigami within the Soul Society, with the Kidō Corps, and Onmitsukidō leaders possibly being on equal status. With one exception, all captains are able to utilize the final stage of their zanpakutō, and are more powerful than even their lieutenants. Knowing bankai gives captains an average of five-to-ten times the strength of shinigami who are unable to perform it. They have also extensively-trained, if not mastered, their bankai, allowing them to actually use the enormous power effectively in battle. Most captains are also skilled in the use of Flash Steps, Kidō, and generally have extensive knowledge of shinigami history and battle tactics. All of these factors create a huge power-gap between captains and lower-ranked officers.

It is not commonly stated but in truth the power of Soul Society lies in the Captains of the Gotei 24, as they are the leaders of its military arm. With them at full strength Soul Society is safe and near unbeatable, if they are broken their strength dwindles making them highly vulnerable. To be a Captain, your character must meet the following requirements
 * Achieved, and possibly mastered, bankai.
 * Passed one of the three different methods of attaining the rank officially.
 * Passed the Captain Proficiency Test, which includes having reached Bankai as a requirement for passing. The passing of the test must be witnessed by three other Captain, one of which being the Commander-General (see above). Most Shinigami become Captains this way, so this is the standard method.
 * Have personal recommendations from six existing Captains and approval from at least three of the remaining seven. So your character would have to be very friendly or very noteworthy. In other words, no matter how strong your character, they likely won't become a Captain this way unless they know how to network or have ideal values.
 * Defeat a captain one-on-one with at least 200 witnesses from the captain's division. Kenpachi Zaraki and Kiyotaka Hashimaru is the only known current captain to have achieved his rank using this method. Due to this, it is heavily advised against, as using this for a character may be considered overpowering (or godmodding, as it's called by roleplayers.)

The Commander-General is the leader of the Gotei 24. He or she is the Captain of the 1st Division. This position is not meant to be used lightly, and using it for a character will put you at risk for criticism from other users.

Vice Captain is the second in command of their respective division a Vice Captain often accompany their captain and is the 2nd seated officer

A Lieutenant is the 3rd seated officer in a division.

Lieutenants function as executive officers along with the Vice Captain for their division, taking care of or supervising day-to-day operations. They (along with their captains) are not assigned to one specific squad. They generally only know the shikai of their Zanpakutō, but are still the third strongest in their division. In the case of a division captain's death, departure, or other circumstances making him unable to perform their duties, the lieutenant acts as the "substitute captain" until another can be assigned. They generally only know their Shikai, but in some cases may know Bankai, but rarely have mastered it.