@Interestedparty
It depends. Chapter 1 will be all about the MC.
Chapter 2 is where you meet the heroes, you will be able to interact with them directly or indirectly.
You could use shapeshifting to disguise yourself as a human who wants to join the party.
Or maybe as a monster, revealing yourself before getting seriously hurt.
You could use illusions to make the heroes get lost, and then jump out of the woods and help them to earn their trust.
Or you could replace the item they are looking for with a fake copy to sabotage their quest.
Elemental Powers! You could attack the enemy they are fighting, or maybe them.
Or you could summon a thunderstorm and then offer your cave as a shelter.
Psychic Powers! You could stalk the heroes, messing with their minds, slowly driving them to madness.
Or perhaps you could modify their quest, or manipulate the mind of the party leader to accept you into the group for who you are.
I’m still thinking about the possibilities, but generally, being subtle will make them see you as an annoyance, while being more direct and extreme will make them see you as an enemy almost instantly.
It may also be possible to earn their trust without using any abilities at all, you just need to be good with your words and actions.
Of course, it will be impossible for the adventurers to trust you right away, because they have done a lot of research before actually hunting monsters and completing quests, so you are pretty much the last person a human would trust.
I don’t want to say too much about it because spoilers are bad, but you might not be alone at this point, and earning their trust could end up getting easier or harder because of this other character.
As a side note: I was thinking about adding a somewhat crazy twist. The Main Character cannot die without losing his/her tails. That means you might be able to let them kill you, and then come back from the dead later, immune to what killed you, and keep stalking them, just like Jason from friday the 13th.
I know, I know, this sounds overpowered, but each time you die, your sanity meter goes down, making the MC more and more unstable and unpredictable, eventually resulting in complete insanity and loss of control.
hmm… sorry for the wall of text, sometimes my keyboard becomes sapient and starts typing words by itself.