3. He's Batman's Man in the Trenches
We all know how heavily inspired Nolan is by the Bond franchise. And Bond has always needed Q (A techie guy even rocked up in Casino Royale, and the gadget he implanted in Bonds arm effectively saved his life by alerting Mi6, so there!), so Nolans Bond, Batman, needs his Q, Lucius Fox. Batman may be more than just a man, but Bruce Wayne is a mere mortal, and Foxs gadgets have saved him countless times throughout the trilogy. Who else could Bruce trust with the fusion reactor? Who else could he ask to operate the vast mobile network needed to track down The Joker? Without Lucius, we would have no Batman as we recognize him. Bruce was ready in Batman Begins, but his symbol was not. Fox gave him the Batsuit, the Tumbler, the Batpod, The Bat, the grappling gun Bruce is a genius, and may have done up the batcave himself, modified his gadgets, but a lot of it was down to Fox in Applied Sciences. One of the reasons I loved The Dark Knight Rises so much was because Fox was given so much to do. He was never given as much screentime as I would have liked in the first two (not criticising Nolan or the films, its just me being picky), but in the finale he was just as much of an eye on the ground as Gordon for the audience witnessing the horrors of BanesGotham. He met the terrifying Bane, whereas even Alfred did not. He essentially took up Alfreds role, because Alfred wasnt there. Alfred accompanied Bruce toHong Kongas almost his accomplice, so he will follow Bruce where he needed him to go. Gordon is an independent agent who doesnt know who Batman really is, and has his own problems. Batman needed someone who knew who he was, who wasnt afraid to get his hands dirty. And he was able to turn to Fox.