FirstGen Zero-Three was one of the early members of the race of sentient mechanical beings known as the Sentries. At some point he was exiled by the other FirstGen Sentries because his views were considered too radical for him to remain. At some point during this long exile Zero-Three attempted to enter the Null's home in subspace and destroy it, but Zero-Three was unable to enter the subspace realm. Afterward it would conclude that the fight against the Null was hopeless.
In 2381, Zero-Three redirected the transporter beam from the destroyed shuttle Holiday, landing the shuttle's crew on it's large surface. He then talked with the crew and revealed the origins of the Sentries to them as well as information about its attempt to enter subspace. They were able to convince it to return to the other Sentries, but as it used the slip space drive it drew the Null into normal space, leading to the final Sentry battle with the Null and Zero-Three's destruction. (TTN novel: Synthesis)