Julian Castro currently the Mayor of San Antonio, Texas. His twin brother Joaquín is running for Congress this year
Very good choice. He's the future of the Democratic Party in Texas - he could be the person to turn the state Blue. Now GOP knows this as well and will do everything within their power to block his rise - if he runs for Governor in 2014 they'll throw absolutely everything at him. If they are successful in blocking him I think the Pres should find him a high profile gig in his cabinet for the last two years to raise his profile that way.
It seems like Mayor Castro will speak on the Tuesday, likely along with Michelle Obama. Wednesday will have Elizabeth Warren and Pres Clinton as the headliners and then Vp Biden and Pres Obama close out the convention at BOA Stadium.
What kind of speaker is Julian Castro? Well he keynoted the Texas Dems convention this year, so I suspect this will pretty much be the preview -