Everywhere I've read has said it's a pain in the ass to do re-rolling on actual phones for this game, especially if you link the game to your Facebook. There's supposedly tons of good units in the game, so I doubt re-rolling is really necessary and have seen the majority of people on forums saying it's not worth it, but there's still some that swear by it of course.
http://appinvasion.com/index.php?forums/Brave-Frontier/ - Link to the only half-way decent forum I've found for the game so far.
There's a way of getting one of two free heroes, depending on iphone or android, through the game's facebook page, by liking some event page. Here's a guide to how to do it on their supposed wiki: http://bravefrontierglobal.wikia.com..._Hero_for_Free