Log Horizon Fun Fact #38:
Rudy is supposedly from a lower-nobility family, but became disgusted by the greediness of nobles and admired the honesty and selflessness of Adventurers, which is why he did his best to act like one.
Well, it worked, just not exactly in the way he had planned.
Also, I don’t know if this is actually set in stone; half of it I heard from discussions on /a/, the part about him coming from a noble family was Isuzu’s guess. There are some parts of the web novel that get cut from the published light novel (such as the entire Isuzu and Rudy going shopping part in the anime; in the web novel, but not in the published one), so it might have been revealed there.