Do you have access to "Xanathar's Guide To Everything"? If so, you should also consider the College of Swords, which is another weapon-based bard college option. It offers the ability to use a melee weapon you are proficient in as a bardic spell casting focus, which works well for bards who want to use melee weapons in one or both of their hands.
However, keep in mind it does not offer the same "Battle Magic" feature as the College of Valor does, which means you will likely be casting and slashing on different turns.
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."
- "Why aren't you smiting?"
- "Why aren't you using Lay on Hands to heal me or pick up an unconscious ally?"
- "Why aren't you using Turn the Unholy to deal with these undead?"
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.
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.
Best Answer
I'm not familiar with Rolemaster, but based on what you've given us, I can say that a Warlock with the Otherworldly Patron of The Great Old One is what you are looking for.
Some benefits of this choice:
Some drawbacks:
Aren't Warlocks evil?
DnD 5e doesn't have alignment restrictions for classes. So you can be a lawful good Warlock if you can justify it. However, most Great Old One's are neutral or evil. But you can still follow an evil god without being evil. In fact, the PHB even states that
So if you have an aversion to being evil, fear not! However, depending on your DM or your campaign, Necromancy can be seen as particularly evil. This is really only true when you're disturbing dead bodies, and not when you cast pure necromancy spells such as False Life. Spells are not inherently evil, but the circumstances around them can be. Strictly speaking, almost everyone in the DnD world (and real life) see disturbing the dead as evil, straight up. However, you might be able to justify it to your DM. Good luck doing that in a party with a Lawful Good Paladin.
Also, there is no Necromancer class either. You can be a Wizard of the School of Necromancy, but that's another question and answer altogether.
How can I enjoy being a Mentalist?
That's a pretty bold question, one that I don't think I can answer objectively. Suffice it to say that a Warlock of a Great Old One is built around mind-manipulation and madness. One of the suggested gods you follow is even Cthulhu, straight from H.P. Lovecraft's Cthulhu mythos, which is the epitome of "Mind-F***". This warlock has all the tools, it is up to you and your DM to make it enjoyable. If you're playing a heavy combat campaign, you might not find as many opportunities to use Detect Thoughts or Contact Other Plane as a more role-play heavy campaign. Either way, however, I can say from personal experience that being able to relay information to the party with one-way telepathy and Devil's Sight is very handy in a combat heavy campaign.