13th level Humanoid that's able to evade a 18th level party? That's a decent CR gulf to span. Classes alone probably won't do it, since classing something to level 13 means it should be balanced against other 13th level creatures, though min/maxing with an emphasis on evasion might slant that.
As a magic item suggestion, give the NPC a unique Artifact of your own creation that gives them additional evasive powers; probably something that locks to their person so high level PCs can't just snatch it from them to make them powerless. If you want them to just be hard to nail down, have it grant etherealness or earth glide (if underground a lot). If you don't want the Artifact to be a loot item the PCs get afterward, you might try something from the Weapons of Legacy supplement, where the item in question is an item that has quests/requirements to use it to its full potential, and the NPC has done the quest(s) but the PCs haven't/can't.
For the Lycanthrope aspect, normally a Lycanthrope build has to use an "animal" as the base creature type. Can you have this NPC through some means be a non-animal lycan with additional abilities? Create a Lyncanthrope build for a Displacer Beast or similar hard-to-nail-down beast; then the NPCs evasive abilities are intrinsic.
That's Not The Intention
The intention of monsters having a block of stats like characters do is so a DM has stats to use when they look up the monster. If I want to put a Bodak into play, I don't want to have to create stats for it every single time. That's what those stats are for - they greatly speed up DM preparation (and improv if things suddenly change mid session and we need monsters we hadn't planned for).
Those monster stats could be changed, if I for example advance the monster using the rules to do that. But they serve as a good base.
The ones you're intended to be able to play as a PC have the "Monster as Character" section.
But...
Spells like Polymorph let you become most of the monsters. Masters of Many Form can Wild Shape into almost anything. Shapechange lets you change into all kinds of things. etc, etc.
On top of that, you could just ask your DM and say "Can I play as this cool monster?" Your DM can make anything happen, no matter how wacky.
Will they? Quite possibly not, as some monsters can be really hard to integrate into a campaign when other people are playing Dwarves. But it's not outside of what a DM can do if they want to.
A friend of mine and I Once came up with a character concept for a two-headed Ogre. We'd each play a head. We had different character sheets, and even different classes. Each turn, we'd decide what we wanted to do. If they didn't match, we'd flip a coin to determine which head gets to act.
Totally not within the core rules, as written. Totally awesome if your DM & table are cool with it. :)
Best Answer
Robin Hood is an iconic scout: moving through the wilderness unseen and dropping arrows through the hapless tax collectors' apple from a hundred paces. He can appear out of the brush, attack, and fade back without the opponent having a chance to reply. Usually scouts are part of a larger group, whether a band of merry men, as advance guard for a squadron of soldiers, or checking the trail ahead for a band of adventurers.
A ranger, on the other hand, is more like Aragorn from Lord of the Rings. He can smell danger, track prey for miles over rocks, or calm a horse with a whisper. A woodsman, usually a loner (though sometimes with an animal companion), a ranger is often at home with a campfire, the stars, and a good knife. His skill set is very similar to a scout's, but with a basis more in nature than in proficiency with stealth or weapons.