Captain Unagi believes in freedom and the rights of the individual to do what they want. He believes in basic order and governance, but doesn't think that anybody needs to be shackled to it. Due to this, he is a fairly laidback, though very bombastic man. He gives orders and he expects them to be followed, but trusts in his men to get it done their own way without needing him to supervise or dictate exactly how they complete their task, leading to some taking advantage of this trusting nature. That said, in normal interactions he comes across as quite gruff, if not rude and overpowering with his strong presence. He is not a man to be crossed, for once you have earned his ire Captain Unagi Tsunami will crush you like his namesake. He is also a natural protector and father to his comrades, making him a perfect fit as the captain of the Seireitei protection division.
Originally came to Soul Society as a baby when his human body died drowning in a shipwreck. He was the only casualty, so his family did not come to Rukongai with him. For many years he grew up with his adopted family of fisherman, who let him run about and do pretty much whatever he wanted so long as he stayed out of trouble. By the time he had become a young man, the massive amounts of reiatsu he contained attracted shinigami, who decided to recruit him into the Shino Academy. His family encouraged it, since living in Seireitei was a better standard of life than even the nicest districts of Rukongai. Unagi packed up, said his goodbyes, and set off to become a shinigami.
His years at the Shino Academy were not the best. Unagi's penchant for personal freedom and resent of being made to do anything he didn't feel like doing did not exactly mesh well with a school with a strict schedule and hierarchy. Eventually he learned to deal with it, but let's just say he has plenty of black marks on his record. During this time, he found that he was fairly well-rounded in his abilities, except for where kidou was concerned. He struggled and struggled but could never grasp the more advanced aspects of the art, or rather he didn't care to. To cope with the stress that despite his great potential he may be forced to drop out, Unagi took to drinking and brawling.
The young shinigami in training setup an underground fight club of sorts. Only fists were allowed, no killing, and of course the most important rule: don't talk about fight club. Thanks to his extracurricular activities, Unagi could cope with the rest of the academy's demands and burn off that extra anger and stress. He kept up his underground fighting ring until the year he was set to graduate when a most unexpected opponent entered the ring. This man was not a student, but a full fledged shinigami. The 8th seat of division 11, the combat specialists. Apparently word had spread out just enough that it caught this man's interest, so he decided he wanted to personally test the mettle of these soon-to-be shinigami.
In a bout that took over an hour with many ups and downs, Unagi finally bested his opponent. The seated shinigami took the defeat in stride, and once graduation time came it was that very same man that sponsored Unagi's entry into division 11. For several years Unagi fought alongside (and oftentimes, with) his fellow battlers of the 11th, but something just never sat right with him. Unlike back at the academy, these fights weren't that fulfilling. Sure, when he was purifying hollows using Otokonoroman he felt alive and invigorated, but the constant bouts and arguments and conflict with his own comrades were just tiring.
Eventually he figured it out. Despite what he had done back in the academy, Unagi didn't like fighting for its own sake. He didn't want to just battle, there had to be meaning behind the clashing blades and crossing fists. Back at the academy he needed something to blow off steam, and he wanted to do it. Now there wasn't any reason for it. Plus, his fellows would talk down to him for his artistic talents even when he beat the shit out of them and that just wouldn't do at all. With this revelation in mind Unagi requested a transfer from the 11th to the 9th.
Years passed by and Unagi rose through the ranks and he gained more and more control of his power. During his time as lieutenant, the then captain of the 9th decided that after 200 years of service it was time to retire. Unagi, having acquired his bankai a few decades earlier, stepped forward for the position. He was evaluated and found to be a fine suit for the job. That was over 100 years ago and since then Unagi has been restructuring the 9th division a bit. In order to maintain the peace he can often be found patrolling the skies above Seireitei in his bankai Boken no Tensen, personally keeping out a watchful eye for trouble makers and invaders. He knows they're likely not around, but he feels that the presence of the great ship gives a certain peace of mind to his charges, and he loves the wind in his hair.
Reiatsu - In terms of pure raw energy, Unagi has some of the most of any shinigami. It really comes in handy with such a large bankai that he enjoys keeping active.
Kidou - Despite his vast reserves of spiritual energy, Unagi is... Not great at kidou. He barely passed in the Shino Academy and even now he's only average at best.
Zanjutsu - In terms of swordsmanship, Unagi is great though he focuses more on raw power than skilled strikes. He's no berserker mind you, but his swordsmanship is definitely geared more toward overwhelming the enemy than outpacing them.
Hakuda - Hand to hand combat is where Unagi really shines through. He is quite agile and well trained in fighting with his bare hands. In fact, it's often his preferred method of combat and few have ever been able to make him fight seriously when it came to a martial arts spar.
Hoho - Shunpo is a technique and style that follows grace and speed above all else. Unagi is better than most shinigami in the art, when if he had to keep up with another captain-level fighter that specialized even a little, he'd find himself coming up short.
Perception - Unagi may only have one good eye, but damn that's one good eye! Since he spends a great deal of his time aboard his bankai and patrols the Seireitei, he's learned to spot trouble a mile away, and in fact has one of the best sixth senses in all of Soul Society.
Writing - As editor-in-chief of the Seireitei News Magazine, the gruff, adventurous, and overbearing Unagi shows himself to actually be talented in the art of wordsmithing. Who knew?