David was a photography student in Los Angeles who saw Angel in action during his early days in Los Angeles. Impressed at the power and authority Angel demonstrated, David went on to set himself up as a faux Angel, creating his own office and going to great lengths to mimic Angel's look (Albeit to ridiculous excess, such as his hair being gelled to a ridiculous spiky appearance that prompted Cordelia to reflect that he could use his hair as a weapon), claiming to his clients that he was the "real" Angel while the Hyperion Hotel was the "secondary" office with a "doppelganger" on staff to draw attention away from him.
David's central problem was his simple inability to understand Angel's true motivation for doing what he did; he fought demons merely because of the perceived "power trip" he felt when he defeated them, failing to understand that Angel was about doing the right thing regardless of the cost rather than the power trip. On a practical level, his lack of knowledge of supernatural lore or Angel's strength could cause him to make mistakes when dealing with clients, such as killing a demon with a gun without confirming whether it had enhanced healing or specific weaknesses that would allow it to recover from such an attack.
Despite some of the obvious flaws in David's impersonation when compared to the original Angel, he only came to the attention of the true Angel Investigations when one of his clients attempted to sell a Tuningas burial stone for an ancient Tuningas warrior, the stone having "recorded" the warrior's feelings of anger at his death and now spilling out across the city, affecting all demons and turning them wild with anger, randomly attacking those around them.
In the final battle to try and recover the stone, Angel nearly succumbed to the stone's emissions along with the other demons, but David—who had followed the group out of guilt over his role in events—was able to bring Angel back to sanity when he sacrificed himself to save Angel from a demon attack. The knowledge that David had finally understood what it truly meant to be him gave Angel the focus he needed to fight off the stone's influence long enough for it to be destroyed, thus ending the wave of hostility ("Impressions").