The question boils down to the title, but the reason I'm asking the question in the first place is a bit unusual, so I will give all the information here in case that helps with answers.
I recently thought of a nice twist for a side character to play sporadically in our campaign, and my DM thinks it's a fun idea, so I want to work on it.
The character (on the outside)
Roland The Slayer is a relatively well-known adventurer, who has solved his fair share of mysteries all around the kingdom. Famous for his hatred of vampiric creatures, he is well liked by nobles and peasants alike, as he has often protected them from the ploys of the creatures of the night. A few times, Roland has unearthed a vampiric conspiracy in the court of the lord while working on a seemingly unrelated task; many others, he has found and slayed vampires and their acolytes who hid in plain sight between the villagers of a town in which he was conducting business (see, it was them, not wolves, the ones responsible for the missing cattle!).
No conspiracy and no vampiric conspirator can slip under Roland's powerful divination skills. He can't be lied to, information can't be hidden from him, and the undead cannot simply avoid his ability to spot them in a crowd.
The character (in reality)
He is not so much a hero as an idiot. See, Roland is obsessed with vampires (for good reason, as they were behind a traumatic event of his early life), and can't help but seeing them everywhere.
When he was a teenager, he heard of people who were naturally gifted at magic, and thought he was one himself. In particular, he is convinced he can use many divination spells: Detect life, Detect thoughts, Detect Evil and Good, Detect Magic… When he "casts" those spells, he can clearly see in his head whether someone is lying to him, or their intentions, or whether they are dead or alive… Unfortunately for him (and often, for those interacting with him), this is only self-suggestion, and the outcome of these "spells" is just reinforcement for his already-held suspicions.
Many of these ploys he has foiled were just figments of his imagination, and most of these vampire acolytes he has slayed were often just regular folks growing more and more weirded out by this obsessive "investigator". As he didn't take it kindly to "finding out" someone was a vampire or working with them, a fight always broke out, and he always came out victorious after stabbing his enemy in the heart with a wooden stake (or cutting their head off). Of course, this meant there was no one to challenge his conspiracy story (which he himself fully and truthfully believes), and since he knows how to tell a compelling story, his legend slowly grows.
The role he will play
The party will meet him in a new city, and they will all join efforts (hopefully) trying to figure out who's behind the vampiric conspiracy dawning on this town, which is what brought Roland to the city in the first place.
Roland will show off his abilities (and his hot temper when he faces someone working for those filthy creatures!) and extract clues from the conspirators at different points, leading the party into a crazier and crazier situations.
The idea is that at some point it will become obvious that Roland is deranged, and is actually not a powerful divinator but an unhinged fella with some combat skill; hopefully this won't happen until after the party has aided him in some dubious tasks, such as killing and disposing of some city guards who were (not) vampire acolytes, potentially tying their fates together.
My problem
It must be credible that Roland can make use of divination spells when the party meets him. He must be at least somewhat competent in combat (for everything to be consistent), while at the same time not relying on any actual magic while fighting (since he can't really use magic; this is a big reason why he believes himself to be a divinator, as he doesn't have any actual magic experience to compare against his "divination").
My DM has given me complete freedom to choose his race, class and background, from the core game or any expansion, whether they are normally available for players or not. Unfortunately, I'm new to role playing games (having played only a few sessions these past weeks), so I can't think of a suitable one and all the possibilities overwhelm me.
The question
Is there any combination of class, race and background (taken from any available) which would be suitable for such a character? If not, as a last resort, are there any already-made homebrew alternatives which would work (asking about something already made as I have zero experience with homebrew and wouldn't know how to make it balanced)?
His level will be 4 or 5, and multiclassing is allowed. We only intend to have him around for a session or two.
My thoughts
First of all, I could make him whatever I wanted and just not disclose anything to my fellow players when they inevitably ask "So what's the class of your new character?" but that's will just raise alarms, which at least at the beginning I want to avoid.
If I claim openly to the players that I am of the same class I will actually play, I might find myself in a serious disadvantage in combat. I can't make him a Wizard, Sorcerer or Warlock, as he can't use real magic; hence he would be mostly useless in combat (at most stabbing with a dagger or hitting with a staff). On the other hand, he cannot be a Barbarian or a Warrior, as it would be strange for him to be competent at divination.
Another option is to claim to be something to the players but actually be something different on the sheet. I thought of finding a class for him which is capable in physical combat, and which has a subclass/background which can make use of magic (something similar to Arcane Warrior). The players would believe him to be of that subclass, while his actual subclass would be another one. Arcane Warrior itself doesn't work as they can't use divination, but that would be the idea.
I haven't considered the previous idea with classes (e.g. have people think he's a Sorcerer while in reality he's a Warrior) because I think it would be too obvious his combat style does not match his claimed background at all.
Best Answer
Thomas Markov's answer is probably the closest you can get with player-focused material. The following is how I would normally handle a short-term character with specific requirements.
Roland uses an NPC stat block
A moderately experienced player will easily determine your class starting around level 3, unless you play extremely suboptimally.
"I'm an Oath of Devotion paladin."
You can certainly have answers for them, but they're unlikely to fool an experienced player for long. If you play in-person or with open rolls online and roll Sneak Attack that you flavor as Divine Smite, everyone can tell that you're rolling 3d6 instead of 3d8.
Instead, embrace that this is an NPC and use an NPC stat block. Work with your DM to adapt an existing stat block to your needs. Veteran or Knight would be a good place to start. If you need a specific ability, bring one in from another stat block or make one up with your DM's help.
When the other players ask what you're bringing, simply say that you're bringing Roland the Vampire Hunter. You or your DM can tell the other players out-of-character that you will be playing this NPC temporarily, thus Roland has a custom stat block.
If you want the deception to stand up to additional scrutiny, add in (fake) abilities to the stat block that you "use" when needed. Add a 3/Day zone of truth, detect magic, detect evil and good, etc. Do they actually work? Nope. But you can say "Roland casts detect evil and good" (or "Roland uses the Detect Bad Things feature") and the DM can respond appropriately. If someone happens to glance at the sheet, everything appears to be in order.
Lastly, if/when things come to blows, I would highly recommend handing Roland back to your DM. PvP is a tricky situation, so it's usually better to let the DM handle it. It helps redirect some resentment to the NPC rather than you, the player.
A warning about secrets
In my experience, player secrets at the game table almost never work as intended. The warlock with Mask of Many Faces expects a dramatic scene when their true face is revealed to the party. Instead, the players shrug and move on. A sudden betrayal often evokes anger at the player rather than their character.
Danger! Every table is different, but this could easily cross a line. It might be a dramatic moment in a TV show, but players are often closer to their RPG characters. Instead of "I can't wait to see how this affects Character X in the next season", many will feel "I just murdered a bunch of innocents because Player Y broke our social contract".
If you and your DM still want to go through with this, I'd recommend strong mitigating circumstances. When all is revealed, you want players to be able to say "maybe they didn't deserve to die, but they were __________" (serial killers, arsonists, people who talk at the theater).
In most cases (though it may not work in your current one), I prefer character secrets. All the players know Character X's secret, but their characters don't know. You might expect that to "ruin the surprise", but I've found that it enhances dramatic moments. The players will often have their characters get close to the truth or almost catch the other character red-handed. They also have a chance to prepare how their character will respond to the eventual reveal.