Describe your own Overlord character, what your floor in Nazarick is like, and what your custom floor guardian is like. Remember every member of Ainz Ooal Gown must be a high-level monster. If you are a custom monster or class, describe it. We're going to pretend that we stayed in game when it shifted reality, and got to play our character for real along with Lord Ainz.
You're allowed to be completely OP because that's the point. Go ahead and create magical items or world items if you want.
I would be a Troll Witchunter. Troll for the high strength, high constitution, and regeneration ability. Witchunters are warriors who specialize fighting against spellcasters. They start out with high magic resistance and gain additional magic resistance each level, and at a certain level gain the Anti-Magic ability, which lets them damage any magical creature or magical barrier using mundane weapons. One caveat: they cannot use magical equipment due to their hatred of magic, and even benevolent spells like healing spells have to overcome their magic resistance to have any effect. Witchunters naturally have a hard time healing after battle as they must rely on herbal remedies and rest, but having troll regeneration helps a lot with the healing problem.
Depending on what the world considers "magical" he probably wears dragonscale, wyvernhide, or chainmail armor, and uses some kind of a battleaxe as his primary weapon. His immense strength also lets him hurl objects for ranged attacks and he likes carrying a few warhammers in his backpack just for throwing at things. He can destroy buildings with these, as they hit with the force of a catapult.
Outside the game my character would have loved trolling forums, and in game he loves arguing with people. Because he's not undead, he gets to feel some emotion but mostly just the bad trolly ones - like anger, greed, and even lust. He loves arguing with Lord Ainz and pointing out all the flaws in his plans (real or not, mostly not) until Lord Ainz decides on something different, and then he'll convince him that the new plan is even worse. (Unless Lord Ainz realizes he's being trolled, and then just tells me to shut up.)
While Lord Ainz pretends to be the hero Momonga and save the world, I would torment the villagers with pranks, such as actually playing the role of a bridge troll and jumping out in front of unsuspecting merchants and threatening to eat them unless they hand over their coins. Why? Because that's more fun than being a hero. If they refused I wouldn't actually eat them, but I would have my guardian rob their wagon while distracted. (because humans taste bad, and because it's more fun to watch them weep and lament their misfortune while I laugh)
My floor on Nazarick would be an indoor garden with trees and all sorts of rare plants. Many of these would be for making medicine, and my character would have trained up his herbal skills to do that. While beautiful, it's beauty is deceptive as I would also keep a menagerie of monsters on this floor, such as wyverns, gelatinous cubes, and mimics.
My floor guardian would be a nymph illusionist named Arixxana. I chose nymph because they are the most beautiful fairy race, and being a total perv I would have given her a stupidly huge bust. Also nymphs hate everything ugly, and since trolls are one of the ugliest monsters in the game, it brings me glee to make her my servant.
Her illusion magic would let her support my character by casting area illusion spells. I could hide behind an illusory wall for instance, since that wouldn't target my character. She could also cast invisibility on herself, or cast charm person to gain control over other people. With high charisma and charm person, she would be a great merchant and get great prices on anything I would wish to sell. She might have utility spells like summoning phantasmal monsters to assist with travel. (a headless horseman, maybe? or a flying carpet?)
My character would have two prized possessions. One is a magical treasure chest of holding, which he keeps in his room and stores all of his ill-gotten gold. The other is a world item that can fully revive a dead target once per day, at the cost of the user's own life. Since my character has such intense magic resistance a resurrection wand would likely fail, but magic resistance has no effect against powerful world magic, which is on a whole different level. This world item is in the care of Arixxana, who would revive me if I was ever defeated in battle. Lord Ainz would probably use a wand of resurrection on Arixxana if this was ever necessary.