Aaron is a Caucasian male with short dark hair and blue-green eyes. He's short, with a slim but wiry build, though the Power Armor he favors makes him look bulkier. With his helmet on, he looks more like a steampunk version of a Stormtrooper than anything else.
The star of an unwritten crossover, Aaron is essentially Eren Jaeger, the main character of Attack on Titan, as a Fallout 3 character. Indecision on the part of his creator means that he has two histories. In one, he is the canonical hero of Fallout 3, having been raised in an underground Vault until his father left, after which he followed him out to the Capitol Wasteland. The other is more of a direct port of Eren's; he lived within the walls of a Brotherhood of Steel outpost, until an especially large Super Mutant destroyed the settlement when he was ten, killing his mother in the process. Aaron vowed to completely exterminate all Super Mutants in revenge. He subsequently joined the Brotherhood, and had just completed his training. Either way, he was exploring the DC Metro when he found himself falling through one of the Wastelands’ many, many plotholes and into the PPC.
If Natasha is used to being the expert, Aaron is used to being the newbie. He knows that he was recently recruited by the Brotherhood of Steel, and has at least one set of memories of going through military training beforehand. This has rubbed off on his demeanor; he tends to be polite and respectful towards those he sees as superior officers, and has a hard time remembering he doesn't need to salute. Over time, however, he's adjusted to the PPC in general and Natasha in particular, and isn't afraid to tease her. He's a little more squeamish about killing people than Natasha, especially if they're not particularly dangerous or evil, and would rather push for recruitment.
In general, Aaron reflects more of Eren's more "typical" character traits. He possesses immense loyalty to those he cares about, both from his time at the PPC and his backstories before joining it, and is more than willing to fight for them, as numerous fistfights with Butch DeLorea can attest to. He has something of a tendency to fanboy the Brotherhood of Steel, and can recite the kill stats of its most prominent members from memory. Though Aaron lacks the more suicidal urges of his canon counterpart – nobody's about to start calling him "Death Drive," especially not at the PPC – he does tend to be unusually reckless when faced with Super Mutants or other large, humanoid monsters that eat people.