A jawline that's out of this world..
Pics is mentally tougher than people give him credit for, to go along with the relentless consistency and fighting ability. It's kind of a Spanish trait when you look at Robredo and Nadal.
I think you are mistaking mental toughness with dogged persistence
I love Ferrer and he is a great player, but mentality has never been one of his stronger suits. he folds under big pressure. however, he is insanely consistent and reliable. obviously that's never been enough in the biggest occasions
he's definitely unique in having such an illustrious career on the the very brink of being at the top of the game. I am not sure I can think of another pro who so consistently stayed within the top 5 or so but never managed to win a slam or break entirely up there. plenty of good players have made it TO the top 5, but generally they do not stay there for such a big portion of their career like David has
and as to what makes him so mentally tough- I don't know you'd have to ask him