Well, I've written a lot of bios for OCs for my friend's RPGs, so I'll give this a shot.
Name: Enkidu Shiina
Gender: Female
Division and Rank: 5th Division, 7th Seat (Temporary Captain)
Zanpakuto: Kamaitachi (Cut caused by a Whirlwind) It's a regular katana with a crescent-moon shaped guard in it's sealed form.
Shikai: "Sundan, Kamaitachi!" (Cut to pieces, Kamaitachi!). This command turns her zanpakuto into a scythe. Its special ability is to boost her speed by manipulating the air molecules around her, lowering and raising her friction as needed. This also allows her to enhance the damage caused by the weapon by adding an air blade to her scythe edge. While it took her years to fully master it, she can boost her speed and the precision of her shunpo to levels previously only reached by Yoruichi, the Goddess of Flash-Step. However, she still has much to learn.
Bankai: Shiina has not yet achieved bankai, but she has been working furiously at attaining it practically from the moment she left the academy.
Kamaitachi appears to her in the form of a weasel, and says that someday she may rival the Goddess of Flashsteps, Yoruichi... But she is not ready.
Appearance: A medium-height, athletically built young woman with lightly tanned skin, long brown hair done up in a loose ponytail with a green ribbon, and cool brown eyes. She wears black, sleeveless gloves to handle her shikai, and always has a few bandages on various parts of her body (cheek, arms, legs) as the use of her shikai always leaves small cuts on her.
History: Cocky, brash, and fun-loving, Shiina is one of many Shinigami to come from the 80th District of Rukongai. A reconnaissance and close-combat expert, she started out in the 11th Division, but soon transferred to 5th at Captain Aizen's personal request. She herself doesn't know, all she knows is that Captain Aizen chose her and she happily accepted, having a bit of hero worship for the handsome captain (though not as badly as Hinamori). It did not become apparent until later than Aizen had needed her abilities. He often sent her on courier missions and 'stealth training' exercises, using his own zanpakuto to adjust her senses to see what he wanted her to see. Through Shiina, he gathered massive amounts of intelligence on Soul Society he later used in his plan to obtain the Orb of Distortion.
Furious at being used, Shiina briefly considered leaving the 5th after Aizen's betrayal. However, with the Division's captain a traitor, and it's vice-captain a mental basket case, she stepped up and assumed unofficial command over the 5th. She has managed it's recovery, training, and logistical needs with 8th seat Umino Tsume, as the higher seats were specifically chosen by Aizen to be nearly ineffectual without him. Not to the extent that Hinamori was affected, but still nearly useless.
Her charismatic spirit and determined nature have endeared her to most of the Division's members, and she has sworn to lead them from the Aizen disaster to a brighter future.
Personality: Despite her beauty she has always considered herself "one of the boys", and only kept her hair long out of respect to her dead mother's wishes. Fun loving and brash, she enjoys training, eating, and parties. Rangiku Matsumoto is probably her best friend. However, she is somewhat insecure about anything to do with femininity, probably why she has not joined the Shinigami Women's Association.
Fighting Style: Shiina relies primarily on her high speed in battle, flash stepping with incredible and deadly precision through enemies and slashing them apart. She can very easily sow chaos in large enemy formations and her combat skills are focused on hit and fade attacks. Not that she can't hold her own going head to head with foes, but she rarely has to. Her main weakness is a near-total lack of talent in kido. She can perform the spells absolutely, positively required for graduation from the Shinigami academy but that's it, and they are so poorly powered she rarely bothers with them anyway. Shiina has learned how to minimize and even mask the sounds she makes while moving with her shikai's abilities, but as it requires a significant amount of reiatsu to do so it's only practical in situations where her opponent's reiatsu sensing ability is useless.
Name: Umino Tsume
Gender: Male
Division and Rank: 5th Division, 8th seat
Zanpakuto: Hogosha Kame (Means "Guardian Turtle"). A fairly standard katana with a turtle-shell shaped guard.
Shikai: "Genni kawa, Hogosha Kame!" (Harden your shell, Hogosha Kame!). Unusual among zanpakuto, Hogosha Kame turns into a large, dark green turtle-shell like shield. It has a few special abilities, the first being it's incredible strength-It cannot be broken, absorbing cero blasts and zanpakuto strikes with equal ease. It appears to work by absorbing the force of anything thrown at it with it's own and Tsume's reiatsu, and redirects some of the force into a pocket dimension, while using the rest to reinforce itself. Tsume has also demonstrated that Hogosha Kame can make for a powerful offensive weapon, learning how to manually redirect some of the absorbed energy to hit enemies with the shield and do more damage.
Bankai: Tsume has only recently begun working on achieving bankai, as it took decades for him to even achieve shikai. However, Hogosha Kame has revealed herself in a few dreams to Tsume, taking the form of a gigantic sea turtle and saying she will appear to him soon.
Appearance: Tall, lanky young-appearing man with messy, short brown hair, green eyes, and thin-rimmed glasses. Rather unassuming looking.
History: Coming from a middle-class family in Rukongai, Tsume originally has been in the 5th Division for most of his career as a shinigami. Well read and educated, he served as the Division's book keeper and took care of the filing system. It was assumed he was there simply to stay out of the way, as everyone else in his graduating class had achieved shikai well before him.
It was not until he was promoted to 12th seat that he began going on Hollow extermination missions and started gaining some confidence, showing a keen tactical mind and cool head in battle. He achieved his shikai just in time to save a family of spirits, which earned him the promotion to 8th seat. During the Ryoka crisis, he fought several shinigami from the 11th, 9th, and 3rd divisions when civil war erupted, non-lethally defeating every attacker. In the wake of Aizen's betrayal, he has acted as the Division's second-in-command, trying to restore morale to the battered but unbroken division.
Personality: Determined, organized, but a bit spacey at times. Somewhat clumsy outside of battle and is constantly losing his glasses. Very intelligent but somewhat reclusive socially, and has mild obsessive-compulsive disorder. Has a massive crush on Enkidu Shiina.
Fighting Style: Often acting as a literal shield, Tsume's fighting style is heavily oriented towards the defensive, however his significant knowledge of kido and decent abilities in hand-to-hand combat allow him to hold his own and cover his allies at range. In close-range fighting he smashes, bashes, and slams his shield into his foes, knocking them back or senseless before finishing them off with a kido blast. He has been practicing with throwing his shield in combat, and while his aim is terrible his zanpakuto always comes back like a boomerang, letting him try again and again until he gets it right.
) ice funnel that he can then fling like a shuriken.