Character Analysis
Fox has a much larger role here than he does in the comics, where he usually just runs Wayne Enterprises while Bruce pummels criminals by the light of the moon. Here, he's kind of Bruce's Q Branch: using Wayne Enterprises money to deliver all kinds of gadgets that Bruce finds ever-so-useful. From a plot standpoint, he answers the practical question of where Batman gets "those wonderful toys" (to quote an earlier movie), as well as demonstrating that Batman can't be Batman without a solid team to back him up.
From a personal perspective, they kind of have a fun vibe going. Lucius knows that Bruce is Batman, and Batman knows that he knows, but they never talk about it directly… though they do talk about it a lot. So it's all hidden phrases and double meanings and winking at each other in the boardroom, instead of Bruce just coming out and saying "I need a cooler Batmobile, buddy." "Three buttons is a little 90s," Lucius smirks at Bruce when he asks about a new suit, knowing full well that he's looking for an entirely different kind of suit.
It's weird seeing that kind of playfulness in a movie so serious, but that's the way their partnership has evolved. Lucius is properly grateful of course—Bruce saved his job and made him the head of his company back in Batman Begins—and since he knows what Bruce does with all the high-tech goodies spilling out of the lab, he must feel pretty good about his place in the universe. No wonder he and Bruce have fun-filled conversations that no one else gets. They're teaming up to put Gotham's scumbags behind bars! Woo-hoo!
At the end of the day, however, Lucius serves a more important role that just filling out the monthly batarang quotas. Like Rachel, he knows where the line between right and wrong is, and he's there to tell Bruce exactly when Batman's crossing it. His take on Bruce's cell phone surveillance system?
LUCIUS: This is too much power for one person.
Luckily for him Bruce agrees, turning the machine over to Lucius and letting Mr. Fox destroy it once the Joker is safely trussed up. Like Rachel, Lucius is good at reminding Batman that he can't live long enough to see himself become the villain, and Batman is smart enough to take the lesson to heart.