Additional Stats

Actual Age: At least 485
Soul Edge stops it's user from aging but will possess them in the process. Soul Edge also is powerful enough to influence those it's connected to, even if they don't use it. For example: Ivy's father was possessed by Soul Edge. She stopped aging in her mid 30's. Decades of caretaking Soul Edge but not using it has resulted in him not aging.
This lack of aging is actually apparent in the games. Other characters like Sophitita age normally. But Voldo has barely changed at all outside of new costumes. The comparison image on right compares Sophitita and Voldo as an example. If he really doesn't age, there's a good chance he's still alive today.
Personality
There's more than just a whole lot of Loyalty in this fellow!
He's a lot smarter than he seems.
Despite lacking all formal education, the games mention he is "self taught" in fighting. This isn't the only thing he's self-taught in, though! The Money Pit is heavily trapped. All of the traps need to be reset and maintained. Voldo had to learn to do this himself. He is mentioned to be able to sense energies and see Vercci's spirit. These weren't common abilities. He had to have picked them up though use and practice. Living alone means Voldo has to tend himself if injured and take good are of himself to avoid getting sick. He is well aware of these things and has had to learn how to do it all himself because...
Voldo is actually cleanly and cares about his health.
Ever notice how he never has eyebrows or any body hair at all? This hints that he isn't bald because of age: he shaves his whole body. Back before lice treatments were a sure thing, people would simply shave their whole bodies to prevent lice. This kind of behavior means he knows that the Money Pit is not a healthy place to live. It's an effort to try to stay clean and healthy. On a more subtle note, his nails are always short and clean. This requires attention to his hands and regular hand washing to do.
The mouth and eye covers actually serve a purpose.
Eyes are a weak point. Even if you can't see through them, eye injuries still hurt. Voldo is covering his eyes to assure that nothing gets in his eyes. The mouth covering is basically a bite guard in most costumes. He's trying to protect his teeth and/or keep dirt and water out of his mouth. He already has shown to care about his health, so wanting to keep things out of his mouth and protect his teeth makes sense.
He also cares about his appearance despite being blind.
Considering that most of his outfits have harnesses and armor pads, it would be very easy to mix and match parts. He could just grab anything and put it on, but that's not how he does things. Voldo's clothing and armor always match. This means he is taking effort to assure that he keeps matching outfits together and is careful in what he wears. He wouldn't bother doing this unless he cared about his appearance.
Voldo knows about and uses intimidation tactics
Not having to fight an ememy saves time and reduces risk to one's health and safety. Voldo clearly has an understanding of how he looks, too. So why not use his appearance to intimidate the enemy? This would explain some of his outlandish costume choices. It also would give a secondary use to shaving his body and wearing eye and mouth guards: it makes him look more intimidating. Choosing strange body movements while fighting disorients and frightens opponents as well. This reduces enemy morale while giving him an advantage in that he's hard to predict. He also frequently makes use of ambush tactics. Ambushes serve both as a scare tactic and a means to get a free hit in on an intruder. Sometimes an ambush is all it takes to scare someone away!
He has to be tenacious, brave, and clever to pull all of this off.
The cleverness allows Voldo to keep learning and advancing his skills. He needs to be clever to keep adapting to his extreme living situation and the many threats he faces. But all the cleverness in the world won't help if you aren't brave and tenacious enough to keep going. He lived in the Money Pit for many years, so we know he's definitely brave and tenacious. As to why he was still there....
He uses acts of service as a love language.
There's many ways to show you care about someone. One way is known as "acts of service": you choose to do things for the one you love. Voldo in canon loves Vercci. So he chooses to serve him wholeheartedly with all of his body, mind and spirit as a way of saying "I love you". I personally suspect it's a romantic love, too. Frankly, people aren't likely to murder a whole lot of people then hide on an uninhabited island for just anyone. And he's been at it for decades! Now that's some serious deep love.
He's really devoted and loving, not insane.
Insanity would mean there's no rational reason behind his behaviors but closer examination reveals that everything has a reason. His strange appearance has a tactical purpose. He is aware of the dangers of his living situation and is taking steps to mitigate risks. And, despite extreme living conditions, he is behaving in a rational maner. Voldo is sane. He's just so deeply in love he looks crazy.
Random extrapolations

Favorite foods: Coconut, especially young coconut or macapuno
The Money Pit was on an island that was far enough south to be able to grow coconuts. Coconuts are nutrious, easy to prepare, and store well. Young coconuts and macapuno are the tastiest versions and both can come from wild coconut trees.
Favorite animals: Cats and rats
When Voldo killed the sailors, he had no reason to harm the ships' cats. Long ago ships always had at least one cat on them for longer voyages. The cats were a source of morale that helped keep rats and mice at bay. The decendants of these cats stayed on the island with Voldo and served as companions. There's also multiple official pictures of him interacting with rats. Rats are easy to tame and make great pets, so he might have had pet rats as well.
Favorite enterinment type: Music
Music is still accessible even to the blind and used to be a special thing you had to go out to enjoy unless you were wealthy enough to have a house musician. His fighting style in-game is marked as "dancer". Dancer... music fan... makes sense. I can picture him especially being into complex melodies (ex: Jazz, trance, metal, classical) as there's more to interact with.
Tea of Coffee?: Tea
After years of being on the island any leftover rations had to have expired. Old hardtack gets musty, turns brown, and has a mealy texture. It's like stale coffee but nastier. The smell of coffee might be OK, but the taste would remind him of rancid hardtack. Tea doesn't tend to go bad over time like coffee can, either. As long as it stays dry tea can keep for many years. If he had anything on the island, it was probably tea. This combination makes him a tea guy.
He probably has PTSD:
He spent decades of being isolated on an island with no human contact except for random attacks. The only times he left the island resulted in him being attacked over and over again. All of this is a recipe for PTSD.
He likes quiet elegance:
His youth was spent serving the very opulent Vercci. Then he spent decades guarding a place full of expensive treasures. He's used to refined elegant things. But the isolation and PTSD make him not do very well in loud, fast-paced places. Quiet elegance makes him feel at home.
Thoughts on Vercci
The source is vague on Vercci, but I have tried to gather things based on what I saw. It mentions that Vercci was a proud man driven by money and power. He cherished things, not people, and frequenty made grand displays of his wealth. He was brutal and cold both inside and outside of business. As long sa he came out on top Vercci didn't care who got hurt in the process. This plus being a weapons collector and seller is why his nickname was "The Merchant of Death." Being a jerk to everyone earned him a lot of enemies. This is what got him killed in the end.Voldo is simply referred to as Vercci's "right hand man". It's alluded to that Voldo's weapons, costumes, and armour were gifts from Vercci. However, at no point does it say Vercci actually cared about Voldo. Occasionally the source mentions Vercci or his spirit praising Voldo for a job well done. But praise does not mean love. The only time I could find Vercci actually showing any caring towards Voldo in-canon was when Vercci's spirit warned Voldo not to touch Soul Edge. That's it. That's all I could find. Most of the time Vercci's ghost barely comminicates if he shows up at all. When he does show up the communication is typically regarding his stuff, not Voldo.
What I think happened
Vercci picked Voldo as his right hand simply because he was really good at following orders, many of which involved killing. Eventually Vercci viewed Voldo as a favored weapon, not a man. As a weapons collector, the idea of having a living weapon beside him proably amused Vercci greatly. Vercci didn't care what Voldo felt; he was just too useful not to keep near. Voldo's fanciful outfits and beautiful weapons were not kind gifts. They were decorations for Vercci's favorite weapon that doubled as a means to show off Vercci's wealth. Voldo was both show pony and status symbol, but never really loved. If anything romantic or sexual happened between them, it was due to easy access or boredom. On Vercci's side it would have been passionless self gratification only. After death Vercci only bothered coming back from the afterlife because he cared about his property. This is why Vercci never spoke with Voldo unless it was regarding his stuff. The only reason Vercci warned Voldo about the evil sword was because "Vercci's Property" in his mind actually included Voldo himself. If Voldo used Soul Edge, it would claim him. Vercci would lose ownership of Voldo. And Vercci hates losing anything.
Unfortunately, Vercci is probably the first guy Voldo ever loved. Voldo didn't know that he should have been treated better. He might even have been too blinded by love and loyalty to even notice he wasn't really loved back. When Vercci gave him things, he thought it was a sign of affection. When Vercci commanded him to fight and kill for him, Voldo did so out of love. He felt that being trusted as Vercci's right hand meant Vercci loved him. He began to yearn to be called upon just so he could show he cared! When Vercci's spirit showed up after death, Voldo took it as a sign of eternal love. It never occured to Voldo that he was being used and treated as property the entire time.
Voldo gave Vercci all the best years of his life and served him with utmost faith, love, and loyalty. And what did he get in return? He was treated like a living weapon and used as a tool for years. Then he was sent to an empty compound on an uninhabited island as part of the weapons collection and/or to guard Vercci's stuff. After death Vercci's spirit would go years between visits with Voldo with zero explanation. This left Voldo confused, lonely, and desperate to be loved. He stayed in a place he knew was a death trap just because he hoped that it would win the one he loved back. It didn't work.
Voldo deserves better.
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