Tatsuo Shimabuku was a karate-ka from age 8, learning Shorin-Ryu. After mastering Shorin-Ryu, he then mastered Goju-Ryu. In 1955, he was chosen by the United States Marine Corps to teach select members his style. One of the first was Harold Long, who taught many people the style upon returning to the US. This led to Tennessee being a center of sorts for Isshin-Ryu. Because of how recent the style is, it was possible to record Master Shimabuku running the katas and as such have an undiluted record of the proper way to do so.
Because of Shimabuku's combination of the two styles, Isshin-Ryu has no 'basic' kata. All of the kata are black belt kata from either Shorin-Ryu or Goju-Ryu.
Seisan is the kata taught to white belts before their advancement. It is considered the simplest kata in Isshin-Ryu; however, it is still complex enough that it is not uncommon to see black belts run the kata in a tournament. This kata teaches how to punch properly and many of the proper ways to turn and kick. It is named after the Seisan stance, which is used not only in Seisan but also often in other kata.
Seiunchin is the next kata, taught to yellow belts along with Naihanchi. It is one of the most solid kata in Isshin-Ryu; the stance for which it is named is deeper than most. It also contains one of the only blind turns in any kata. Its bunkai differ upon whom you ask, but it is an example of some potential grappling set-ups and most importantly, solidness which is from Goju-Ryu.
Often mispronounced "Na-han-chin" in the US, this is the shortest kata in Isshin-Ryu. The Naihanchi stance is tight and the bunkai often consider the fighter as having his/her back against a wall. The stance is considered awkward by most and Naihanchi is almost never seen performed in tournament for these reasons. When it is, it is almost always by a black belt.
Wansu, sometimes called 'the dumping kata' by students, is the fourth kata learned. It opens solidly and contains many sharp motions; with practice, it can be one of the most impressive kata in Isshin-Ryu, especially in a tournament setting. It teaches more advanced movements and combos, as well as the 'dumping move'. It is often seen performed by green or blue belts in tournament.
Chinto is notable both for its common turns (throwing a disoriented karate-ka off track easily) and for the fact it is performed at a 45-degree angle. Many advanced movements are performed in the kata, and many grappling set-ups are contained within simple movements. Although the turns can be disorienting to the learner, it is an exercise in learning how to turn effectively and keep on an angle.
Kusanku, though commonly referred to as as 'night-fighting' kata, is not. Instead, it is a better exercise in strengthening concepts learned in Chinto and Wansu especially. The lunging stance is used a few times. Some practitioners complain that the kata is hard on the knees and many with injuries or bad knees cannot perform the kata at all, but it is very powerful and is often seen performed in tournament by brown belts.
Sunsu is one of the few katas only performed by one style. Created by Shimabuku before he created Isshin-Ryu, Sunsu is a very strong kata, combining many of the motions used in other kata. It is one of the more common kata ran in tournament by higher-level brown belts or black belts. Due to the fact that it is a combination of movements used in other kata, it is very easy to make the 'mental switch' into another and mess yourself up.
Used as an exercise in breathing, strength, intensity, and muscle tone, Sanchin is arguably the strongest kata in Isshin-Ryu. The kata is performed while tense, and is run slowly. Because of this, many male karate-ka will run the kata shirtless while in order to show off the muscle tone and intensity; though this is not as common in tournament it still happens.
Isshin-Ryu, as one of the newest forms of Karate, is more malleable than most. It is not uncommon for a more open-minded sensei to say something along the lines of, "This bunkai is up to you, but this is what Master Long always told me it was." Different senseis will tell the practitioner different bunkai all the time.
Next is the fact that Naihanchi is shortened in respect to many other disciplines and begins looking left instead of right.
Finally, the vertical punch. The vertical punch is formed by curling all of the fingers (pinky first, then ring finger, and so on) and then putting the thumb on top of the fist. This is done so that no matter what the distance a punch can be performed with roughly equal power, as opposed to a corkscrew punch which must hit at full extension for full power. Some also suggest that the thumb placement strengthens the wrist. This punch is performed striking with the index and middle finger knuckles and snapping back.