Sprite

A sprite that has chosen to help the party is a forest's vetting committee with a bow. Sprites do Heart Sight before they do anything else, and the party only gets the alliance if they pass the alignment read. Run that as the actual first scene: a sprite lands on a low branch, asks each PC for permission, and uses Heart Sight (DC 10 Cha save, automatic failure for Celestials, Fiends, Undead) to learn their emotion and alignment. The PC who refuses the save reads as suspicious, the PC who fails it reads as honest, and the sprite decides from there.

In play, the sprite is a scout and a peacemaker. Fly 40, Stealth +8, at-will Invisibility on itself. It vanishes the moment it isn't actively talking and never lets the party hand it gold. Common, Elvish, and Sylvan mean it bridges between human towns and the forest factions that won't talk to PCs directly. Use it as a quest-giver in motion, not a sidekick.

In a fight on the party's side, lead with Enchanting Bow. The +6 ranged attack does 1 piercing and Charms the target until the start of the sprite's next turn, which means the next attack against that enemy from anyone else has Advantage. The sprite is not the damage dealer. It sets up the rogue. Save Needle Sword for cleanup on Bloodied targets the sprite can finish without provoking. With 10 HP and AC 15, one missed Stealth check ends the sprite's day, so it should never be in melee that the party didn't choose for it.

Have the sprite refuse to enter buildings. Roofs over its head are a fey objection, not a tactical one. The party walks into the inn alone while the sprite waits on the chimney, which makes its return a small relief every single time.

A hostile sprite means the party did something the forest noticed. They cut a sacred grove, they brought iron into a fey court, they killed something the sprites considered a friend. The sprite alone is a CR 1/4, but it never travels alone in this framing: assume four to six sprites, all invisible, all firing Enchanting Bow from 40-foot range while the party flails at empty air.

Open from Invisibility. The sprites' first round is six Enchanting Bow shots at +6 from concealment, each Charming a different PC for one round. Charmed means each affected PC has Disadvantage on attacks against the sprite that hit them and cannot target it with harmful effects. So the entire party is, briefly, unable to retaliate against the people shooting them. The sprites use that round to reposition to new vantage points, then drop Invisibility (which ends when they attack anyway) and scatter for the next volley.

If a sprite gets pinned in melee, it goes Invisible again as an action and flies straight up. With fly 40 and Stealth +8 against passive Perception checks, the party loses tracking immediately. Sprites do not last-stand. Once half the troop is down, the rest vanish into the canopy, and the encounter ends with a Sylvan voice from somewhere above the party, telling them what they did wrong.

Roll the Charisma saves against Enchanting Bow without telling the players which sprite shot them. The party should never know who they're trying to hit, only that the woods are angry.