Volke

Well, last I heard if you play through the game twenty times, at least one playthrough being on Difficult, without any characters dying in any of the playthroughs, you can unlock him in the Trial Maps. I doubt it's true though.

With the right equipment it's possible to unlock him in the main game by messing around with the coding. Greil was originally going to be a playable character, but was instead killed off for a more emotional storyline. The playable version of him, including his S ranked axe, Urvan, is still buried in the game's coding.

Alongside Greil, there were other things edited out of the final version of the game that are still in the coding. Most of these are weapons:

Wishblade (S ranked Lance that appears in the game but can only be obtained in the Trial Maps)
Urvan (S ranked Axe)
Rexcalibur (S ranked Wind tome)
Rexflame (S ranked Fire tome)
Rexaura (S ranked Light tome that appears in the game but cannot be obtained)
Devil's Axe (I have no idea what this is supposed to be like)
20 HP healing item (it is thought that these were Vulneraries, as in Fire Emblem: Radiant Dawn Vulneraries heal 20 HP instead and Herbs actually heal 10 HP)
Dark magic and Dark Mages (a Sage that used Dark magic was originally going to be the boss of one of the later chapters)
Proper promotion for the Thief class (in other words, it meant Sothe was originally going to have the ability to promote. Why they removed this feature, I'll never know, but I know that Sothe was originally going to be removed from the final version of the game until they decided he would play a more central role in Radiant Dawn)
Shiharam recruitable in the main game (now available in Trial Maps only)
Two more unknown recruitable characters (nobody has found out who they were or whether or not these two re-appeared in Radiant Dawn. There was a rumour that they wre caineghis and Sigrun, but this was latre proven to be false. An unsolved mystery...)