Let's start with the crystal of air jobs, since they're the basics and the first set. It consists of: warrior, monk, white mage, black mage, thief, red mage, and onion knight. It's not clear what exactly were they like in life, but it's safe to assume that they have the affinity to the crystal of air. Let us interpret these jobs, shall we?

The jobs

The warrior could be a freelance mercenary, traveling around the floating continent and hired to do jobs like fighting monsters. I can imagine the warrior being a hardened type of guy, who doesn't care about who's paying him so long he can finish a job. His cause of death may have been that he attacked a lot of monsters head on in Altar Cave, only to be eaten alive since they outnumbered him.

The monk could have been a traveling martial artist, honing in his skill while aspiring to be a black belt just like his master. I can imagine the monk being a hotblooded type of guy, helps travelers but is eager for a fight. His cause of death is being attacked by the undead in the Sealed Cave, where the djinn boss lived in. No doubt that his skeletal remains became undead (possibly through the powers of darkness the djinn wields), or found somewhere in the cave.

The white mage must have been a traveling healer, wanting to heal wounds in the floating continent. I can imagine the white mage as being a caring and tenderly type of woman, the way she uses her magic to mend wounds and cure ailments in villages. Maybe she aspired to be a devout (an expert of white magic) like her master? Know one knows. Her cause of death is being attacked in a forest near Castle Sasune, and since she only used up her healing spells and the wind element aero, chances of her survival is slim.

The black mage is a mage from the continent of Dalg, who came to the floating continent to seek any spells that he hasn't heard of. I can imagine the black mage being a no nonsense and knowledgeable type of guy, who doesn't take anyone's crap and prefers researching spells. Perhaps he wanted to be like Doga, a powerful wizard who trained under the legendary sage named Noah. His cause of death is being attacked by the undead in the Sealed Cave, with him using every last spells to clear out the undead horde only to fall when they outnumbered him.

The thief must have been just a regular thief who steals from Kazus and Ur, and storing the stolen things in the upper level of Altar Cave (that explains why there's treasure chests in the upper floor). I can imagine the thief being a greedy yet helpful type of guy, the way he steals from people and gives some of the things he had stolen to the less fortunate. His cause of death is simple: he was eventually captured by the kings guards and was executed in the castle.

The red mage must have been a mage from Castle Sasune, capable of using black and white magic as well as swords. Every soldier of the castle wanted to be a red mage, and they looked up to him the way he uses magic and swordsmanship. However, when the djinn first cursed the kingdom, the red mage is eager to save the kingdom before the warriors of darkness were to arrive. I can imagine the red mage as a cautious and prideful type of guy, the way he is being careful and uses healing magic to heal his wounds before resuming combat. His cause of death is facing the djinn in the Sealed Cave, and failed. The djinn crushed him before burning him alive, pretty much using up his spells when fighting without any support. The sword, Wightslayer, and the blizzard spell may have belonged to him in the castle.

The onion knight must have been a kid who lived in the village of Ur, who wanted to be a hero long before the orphans were born (specifically, the orphans from the Nintendo ds). I can imagine the onion knight being rather naive and adventurous type of boy, who explores every nook and cranny to see any secrets. His cause of death is when he entered the Altar Cave to explore, only to be stabbed to death by goblins. This cuts his journey short.

Conclusion

I can assume that each of these people have their spirits and their abilities be absorbed by the crystal of air because it knew they'll have an important role for the warriors of light. Perhaps their memory and their profession created the jobs inside the crystal, much like the heroes from final fantasy 5 where their spirits reside in each crystal to bestow a job to the protagonists. As for the onion knight that died, perhaps his spirit resides within the shard of the crystal of air just waiting to be found. When the crystal of air bestows its blessings of light to the warriors of light, then the spirits would lend a hand by having the warriors of light use their jobs and abilities to help them stop the darkness.

Keep in mind that the crystal of air jobs have been around decades before the warriors of light were born, and these speculations also can expand the story.