Is there a true definition?
Leader by performance, but not much of a talker? Sandberg, Dawson, Lee Smith, Maddux & Derrek Lee come to mind.
Leader by having a fiery attitude? Bowa (player/manager), Piniella & Durocher.
Combination of both? Performance & emotion? Grace, Buckner, Zambrano and dare I say Dunston?
Leader in the clubhouse? And do we even know what that entails? Gives pep talks? Takes a rookie under his wing and helps him out? Hell, many of the young guys thought Daryle Ward was a good clubhouse leader.
Since he was a rookie, he couldn't be considered the 'team leader' but maybe as early as next season, Geo Soto could easily become the face of the Cubs. First, he's the 'quarterback' on the field. He's already been extremely impressive on how the pitchers listen to and work with him.
I know I know...he's only been around one season. But you can sense the leadership qualities emerging on the field and in the dugout. I see lots of Pudge & Pudge, Jr. qualities in him.
Although he has the longest eyelashes in MLB history, don't let the Mike Piazza/Richard Grieco looks fool you. He looks like a guy that could be wearing the 'C' for captain for a long time.