Larry is a janitor at the Pentagon. He is generally thought to be senile, but has top-level security clearance anyway because he's adorable. He does look like Jonathan Katz, after all.
Dreamwave comics continuity
When Spike Witwicky was taken to the Pentagon by Hallo, Larry encountered him in the hallways and incoherently rambled at him. Later, as Spike was imprisoned within the building, Larry passed a note underneath his cell door, telling him to be ready at 3 o'clock.
For some reason, Spike assumed that the unsigned note was from Hallo, and signified the time when he was to be executed. Thus, he was surprised when Larry came to the door and snuck him into a broom closet. Once there, Larry gave him a folder of Project Lazarus documents, explaining the machinations of Hallo and Lazarus.
In gratitude, Spike beat him up.