Headcanons

Medical/Magical/General Great Sage Stuff

• Canonically, Rezo knows not only white magic, but the broader shamanistic magic and dark magic as well. He doesn't know holy or fusion magic because he spent his entire life inside the Mazoku barrier, and he doesn't know the Giga Slave, Ragna Blade, or any other spells that call directly upon the Lord of Nightmares.
• He is also an expert on things like herbalism and anatomy... by the standards of a pseudo-medieval world, anyway. So he's not clear on what a germ is but he does know that washing up before surgery reduces the chances of the patient dying from a horrible infection later on, even if he and other people only have vague ideas as to why.
• He's also studied a ton of other random subjects (he knows how guns and bombs work) and while sometimes this is due to his desperation to cure his blindness a lot of it is that he's just an inquisitive person by nature and he likes knowing how things work.
• As a teenager he developed a shamanistic spell that allows him to read with his fingers by raising the ink into small-but-tactile ridges. The downside of this spell is that if he's careless he can cause actual damage to the books by doing this, as well as the sort of damage that naturally would come from rubbing your oily fingers all over the paper. (Mostly a concern when he's handling Ancient Tomes Of Wizardly Lore, which tend to be delicate things.)

Greywords Family Stuff

• He was essentially raised by the church in the city of Sellentia, in the Kingdom of Ralteague; as an infant and young child he lived as a foundling in the orphanage, and when he was around ten he was taken in as an apprentice priest at Ragradia's shrine due to talent for magic.
• His first name, Rezo, was given to him by the staff of the orphanage. He picked out his own surname later in life.
• The priests and priestesses who volunteered at the orphanage were by and large good people who did their best, but they did not have a lot of resources nor necessarily the knowledge needed to adequately meet everyone's needs. Child neglect, malnutrition, and high infant mortality rates were all prominent issues while Rezo was growing up, and his childhood experiences would later go on to influence his charitable pursuits.
• He never learned the identity of his birth family but he was adopted as a ward by the head priest of Ragradia's shrine in order to streamline the process of making him an apprentice.
• I actually don't take it as necessarily canon to the anime that Rezo killed Zelgadis's parents, partly as a personal preference (a fanfiction that has been very influential on my take on Rezo goes with that not being the case, and watching the anime I got the impression that Rezo was generally too pragmatic for that to be entirely IC) and also because the anime tends to be less dark in general than the novels seem to be. But I'll generally either be vague about it or go with whatever any RP partners prefer. It mostly just affects the nuances of his dynamic with Zelgadis.
• Zelgadis is not certain whether Rezo is his grandfather or great-grandfather. Rezo blithely assumes that Zelgadis is fully aware that Rezo is his great-grandfather. They have never had a conversation clarifying their genealogy before 1) Rezo ruined Zel's life and also their relationship and 2) Rezo died (twice) so Zel will most likely go to the grave not knowing exactly what the deal was there. 
• On a related note, Rezo doesn't talk about his personal life much without strong prompting, partly because he's a naturally private person, partly because it just straight-up doesn't occur to him, and partly because Lol What Personal Life
• Once when Zelgadis was like, five, Rezo caught him competing with other kids over how long they could stare at the sun without blinking, and Rezo sternly told them off for it, warning that if they kept it up they could go blind, JUST AS HE, REZO, HAD!!! and for a while afterwards Zelgadis straight up believed that his great-grandfather went blind from staring at the sun for too long.
• Rezo was actually a decent caretaker to Zelgadis when he was a child, if a touch neglectful- albeit more by our standards than by the standards of the culture they live in. There were a few warning signs that Rezo wasn't entirely stable before the chimera incident happened, but the signs were subtle. After the incident, Rezo became increasingly distant and psychologically abusive towards Zelgadis; the mistreatment basically started very suddenly and rapidly got worse. 
• Rezo fortunately wasn't monitoring Zelgadis very closely however, as he was entirely preoccupied with his own research, and many months at a time could go by without the two of them interacting.

Shabranigdu Possession Stuff

• Up until shortly before he died for the first time, Rezo had no idea Shabranigdu was sealed inside him, or that this was the cause of his blindness, although he did strongly suspect that his blindness was a magic-related ailment.
• Shabranigdu's influence is fairly subtle and mostly affects Rezo's moods, unconscious thoughts, and things of that nature, rather than Shabranigdu directly speaking to Rezo. Rezo himself is not sure just how heavily Shabranigdu has or hasn't affected him.
• He sometimes dissociates so heavily that he essentially blacks out and goes through long periods of time where his judgement and moods and such are even worse than normal, and afterwards is unable to remember what happened during these periods. This isn't a case of Shabranigdu taking conscious control, but is a side effect of having Shabranigdu attached to himself, as it's an extreme stressor on his psyche.

Misc Headcanons

• He collects dolls, stuffed animals, and similar toys. He sometimes uses them for practicing enchantments and such but he also just genuinely finds them charming. He is, however, aware this is considered Somewhat Weird And Creepy for a 100+ year old man to do so he's not exactly open about it.
• He... sort of knows how to cook. He basically has a handful of recipes memorized but beyond that tends to go into a state of "Uh," and has a bad habit of either forgetting to eat until he's on the verge of passing out or relying on one of his more-loyal-than-he-probably-deserves retainers to toss some food periodically in his direction.
• He can't understand most sign language but he did painstakingly learn fingerspelling so he could communicate a little better with deaf patients and also just as a like, flex.
• He was semi-oblivious to Erisiel's affection for him, and was under the misapprehension that her feelings were essentially professional admiration to a degree high enough to border on a crush. He has no idea what happened to her, Copy Rezo, or Sairaag after he died.
• Rezo was aware of who Lina Inverse was before he met her (and he had also heard of Luna Inverse and that she was a Knight of Cepheid) via intel he'd gotten from his henchmen; at that point he'd mostly retreated from the public eye but he was still trying to keep abreast of current events and information that might be useful to him, and a child prodigy, related to the Knight of Cepheid, who earned a color from the Sorcerer's Guild as young as Lina did was mildly notable. Particularly one who also had a reputation as a bandit killer.
• Rezo's world doesn't have Christmas but if it did he would hate it.

Contemporary/No Magic AU for Memes and PSLs

• Rezo is a blind pharmaceuticals researcher who has spent most of his career working at a company called Shabranigdu. In between research he's also written a few hefty medical books and papers and he's gotten various professional accolades for his work.
• He's fifty-something, maybe? Older than he looks.
• By default, Zelgadis is his son instead of a great-grandson, but I'm willing to be flexible on this point (or any other parts of their shared history) if I'm bouncing him off of a Zelgadis muse. Rezo did... *something* to Zelgadis that destroyed Zel's trust in Rezo and left physical scars that are difficult to hide. Zelgadis is currently a college student living away from Rezo and the two of them barely speak.
• Rezo used to be a much more upstanding sort, but after he got a job with Shabranigdu it turned out the company was involved in organized crime and other shady activities. Still, Rezo is reluctant to quit out of fear of potential retribution from his employer, a sort of "sunk cost" mindset, and the belief that he'll be unable to find a decent job that accommodates his disability if he quits. He has been with Shabranigdu for about twenty years and became aware of and involved in its shady side at a gradual enough pace that it was essentially a "boiling the frog" process.
• Probably a "functional alcoholic" or has various other unhealthy coping mechanisms to try and deal with the stress of his job. Nevertheless, he maintains an outwardly polite and affable demeanor.
• Usually dressed in a semi-formal style with vest and tie. Think a slightly uptight librarian kind of look. Very dweeby.
• Does indeed hate Christmas.