We only see Nenjiro from Nenji's painful memories, but what we do learn is that he was a very devoted and brave navi. Nenjiro worked alongside Nenji in making the best fireworks possible, to acheive their dream of creating a firework so grand that it can be seen around the entire world.

Unfortunately, when a firework launcher went out of control, Nenjiro sacrificed himself in order to stop it. This left Nenji broken hearted, and when Napalmman came along sometime later, Nenji temporarily gave him the name of "Nenjiro", proving he was never forgotten.

Nenjiro's only and final scene was really touching, you have to feel bad for the poor guy.
This character description was made for episodes that I have seen so far. It may be updated as more episodes become available.