District attorneys in Japan serve a very unique position in that they can make or break a case all by themselves. Ideally, a district attorney will not so much as mount a case if he/she finds there's not enough evidence to support it (in this regard they're not very different from prosecution in other countries) or if he/she doesn't believe in the defendant's guilt. As a general rule, when it comes to the criminal process in Japan, the defendant is usually better off pleading guilty and hoping for a lighter sentence (interesting fact: the prosecuting phase and the sentencing phase are carried out separately in Japan. In both, the prosecutor and the defense attorney argue).

However, I'm not sure if district attorneys can serve as defense attorneys in Japan. (Most of the time, they'd simply not want to.)

I smell court law plawt.