c´est simple y suffit de chercher un peux sur game faq http://www.gamefaqs.com/pm/portable/ds/data/929449.html :
In Ichigo´s story mode, you will come to a point where you have to face Renji and he acts as a ´checkpoint´. Depending on what you do, the story branches out.
Renji will tell you to defeat him while you have...
- A full special Gauge (I usually have it at 3)
- A low special Gauge (close to 0)
- Anything else (probably range from 1 to 2)
(These are highlighted in his speech text before you face him.)
Depending on what you do, you branch out and face different opponents who offer you their own special conditions (e.g. against Kurotsuchi, you cannot use the "B" button dash). If you fail any of the conditions, you will loop back to Renji and have to try again.
Basically, going on different paths enables you to meet different characters (some unlockable). Just keep experimenting and you´ll get to the end. Once you finish Ichigo´s story, you will unlock some of the other characters´ scenarios. All this is base on memory, so if I have made a mistake, feel free to correct me. Thanks.