Elemental Affinity: The Yule Lad has an affinity towards cold. It gains immunity to this damage type. 

Elemental Aversion: The Yule Lad has an aversion towards psychic and fire. The Yule Lad preferentially avoids sources of these damage types.

Bitter Aura. While it's alive, the Yule Lad generates a bitter aura that fills the area within 10 feet of it. At the start of the Yule Lad's turn, all nonmagical flames in the aura are extinguished. Any creature that starts its turn within 10 feet of the troll takes 10 (3d6) damage of the Yule Lad's elemental affinity.

Regeneration. The Yule Lad regains 10 (3d6) hit points at the start of its turn. If the troll takes damage of a type listed in its Elemental Aversion, this trait doesn't function at the start of the troll's next turn. The Yule Lad dies only if it starts its turn with 0 hit points and doesn't regenerate.

Invisibility:  The Window Peeper is invisible.

Thief in the Night: While a creature is performing a short or long rest in range of the Window Peeper, the creature must roll a DC 10 Wisdom (Perception) check once per hour. On a failure, the Window Peeper steals a random magical item from the creature (common, then uncommon, then rare), if any are available. 

Greedy Thief: If the Window Peeper encounters a creature with only rare magical items, it will attempt to steal the item regardless of outcome of the perception check. On a successful check perception check (DC 10), the Window Peeper's invisibility ends.

Leppaludi's Upbringing:  Leppaludi isn't a model father. Due to childhood exposure, Yule Lads have resistance to fire damage.

Bahumbug (3/day):  Their joy is your lack of such. Yule Lads can cast Dispel Magic at will at 3rd level.

Legendary Resistance (1/Day): Whenever the Yule Lad fails an ability check, it can choose to succeed instead.

Multiattack. The Yule Lad makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 6 (1d6 + 2) piercing damage plus 12 (3d8) damage of its Elemental Affinity.

Claw. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 10 (2d6 + 2) slashing damage plus 9 (2d8) damage of its Elemental Affinity. If the target takes any of the elemental damage, the target must succeed on a DC 15 Constitution saving throw or have disadvantage on its attack rolls until the end of its next turn.

Distract: The Window Peeper casts an item of rarity rare or greater a distance to a spot it can see in range (120 feet). All creatures within a 300 foot radius of the item must make a DC 15 Intelligence check or be forced to move toward the location the item was thrown. Creatures affected in this way must use all movement, including the dash action, to reach the space. On each turn on which they haven't reached the space, they may repeat the Wisdom save, on a success, they can again move in any direction.

Legendary Actions (3/Round):

The Yule Lad can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The Yule Lad regains spent legendary actions at the start of its turn.

The Trickster's Due (1 action): The Yule Lad can take an additional reaction this round.

Misty Step (1 action): The Window Peeper casts misty step.

Winter's Grasp (3 actions): Wintery tendrils grasp at a creature within 120 ft.  The creature must make a DC 15 Strength saving throw or be subject to the effects of the Earthbind spell. An area of 30 ft centered around the target becomes difficult terrain for the extent of the effect, which the Yule Lad ignores.

End of Merriment (3 actions): The Yule Lad casts Dispel Magic as a 3rd level spell.