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Johnny was a UED smith who was guarding the psi disrupter on the fatal day when Vice Admiral Alexei Stukov was killed by Samir Duran. Shortly after the death of the Vice Admiral, the zerg invaded the Disrupter, which had also been set to overload.
Johnny was unlucky and was killed by the zerg. He was later found and recognized by a marine who was disgusted by the sight of Johnny, or what was left of him.