Dual Nature. Window Peeper counts as both an aberration and a fey for the purposes of spells and other effects. 

Fey Ancestry.  Window Peeper has advantage on Saving Throws against being Charmed, and magic can’t put it to sleep.

Petrifying Peepers. If a creature starts its turn within 30 ft. of Window Peeper and the two of them can see each other, Window Peeper can force the creature to make a DC 12 Constitution saving throw if Window Peeper isn't Incapacitated. On a failed save, the creature magically begins to turn to stone and is Restrained. It must repeat the saving throw at the end of its next turn. On a success, the Effect ends. On a failure, the creature is Petrified until freed by the Greater Restoration spell or other magic.
A creature that isn't surprised can avert its eyes to avoid the saving throw at the start of its turn. If it does so, it can't see the Basilisk until the start of its next turn, when it can avert its eyes again. If it looks at the Basilisk in the meantime, it must immediately make the save.

Shape Changer. As a bonus action,  Window Peeper can change into a small humanoid no taller than 4 feet that it has seen, or back into into its original form. 

Multiattack. Window Peeper makes 2 Claw Attacks

Claw. Melee Weapon Attack: +4 to hit, reach 5 ft., 1 target. Hit: 8 (1d6 + 4) slashing damage and 5 (1d8) psychic damage.