Some TMNT stuff really isn't for little kids.
Some years prior, Ace was a combat pilot who was a member of a special unit that fought alongside the Neutrinos against the Utrom invaders. After his unit was shot down, his comrades in Infantry X-5 were executed mercilessly by Krang despite their surrender.
Ace escaped this fate, and eventually ended up on the planet Palmadise, a planet controlled by criminal cartels, where he busied himself destroying weapons belonging to the crime bosses. This led to a bounty being put on his head by Papa Chum, and he was nearly apprehended by bounty hunters when the Turtles found him. At first he dismissed them as yet more bounty hunters, but changed his mind when he heard that they were gathering witnesses for Krang's trial.
However, escape from Palmadise turned out to be a problematic affair, especially when the assassin Hakk-R arrived and attempted to kill Ace. He escaped, seemingly leaving the Turtles to fight the assassin, but in fact had merely retrieved their ship Resistance One in order to more effectively fight Hakk-R.
Despite the importance of his testimony, Ace was never able to testify against Krang. The Malignoid Swarm attacked during the trial, and Ace flew a fighter spacecraft in order to defend the Neutrinos. This service was deemed a conflict of interests, and he was no longer eligible to testify.