Raising General Leo 
Considering how brief his life was, there's an awful lot of conjecture as to his death.  After just one battle, he is knifed by Kefka and his character is buried.  His is the only recognizable grave in the game: not even Elaine and Owain (Cyan's family), nor Baram (Shadow's partner in crime) have graves anywhere that you can reach.
There have been more than a few theories on his resurection.  One way involved finding the Cross, and giving it to Ultros at the Colloseum, who trades it for the Scorpion Doll, which can be used to raise the dead..  Another way involved fighting the dinosaurs in the forest north of the Veldt until the Golden Dragon shows itself, then beating it for the Revive potion.  My personal favorite was where you bring the robot from the Magitech Research Facility to its duplicate in the Fanatic's Tower, and have them merge, then bring the result to Leo's Grave.
There is, however, only one way I've ever been able to pull it off:
DD5C-740D

It's a Game Genie code.  Once you get Umaro and Gogo (it looks like everyone else is optional, except maybe Shadow), you go to the airship and talk to anyone.  They ask if you want to change parties: before saying Yes, turn ON the code.  As soon as it's offered you the selections, turn it OFF.  Trying to check status with the code still running crashes the game instantly.
For those of you who are about to remind me that you DO need all characters to summon Leo, look HERE.


A few weird side effects of this code:
 
Kefka appears next to Leo.  For some reason, he uses Terra's portrait, and I'm told if he gets into a fight the game crashes.  I suppose this proves that the code doesn't trigger a set of preprogrammed events, rather it just hacks the units into your party. 
That or the programmers were idly contemplating adding Kefka as a playable character for part of the game, and decided against it.... 
Also, since they replace the two blank slots, they don't count as real units.  That means you can't take Leo in Cyan's dream, Leo can't fight in the Colloseum (he tries, but doesn't show up), and I assume if they're the only surviving member of your group, you're annihilated.  Haven't tested that last one out yet, though...
 
Leo is poisoned.  Apparently the battle against Kefka still counts, and he retains his old stats: as a result, that last poison still appears on his record, and he's slightly weakened.  Notice also that Leo's 3 levels over Kefka: if Kefka was ever going to appear in the game, it's likely he would have accompanied the parties to Thamasa.  Maybe even as a replacement for Relm...and good riddance.  While playing through, I had the misfortune of Relm getting immediately Stone'd by Ultros...with predictable results. 
Relm's special ability:
GLITCH THE GODDAMN GAME!
And for you die-hard Leo fans:
Leo on the Stats page
Terra finally makes some sense
Leo rising from the grave!  Woooo!
Leo...umm...paying his respects....to himself...
Deravonese Version