Instead, embrace that this is an NPC and use an NPC stat block. Work with your DM to adapt an existing statblockstat 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.
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.
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.
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.