Shapechanger. The Undine can use its action to polymorph into a Small or Medium humanoid female, or back into its true form an amorphis water blob. Its statistics, other than its size, are the same in each form. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.

Invisible in Water. The Undine is invisible while fully submerged in water.

Amorphous.  The Undine can move through a space as narrow as 1 inch wide without squeezing.

Soul binding. If an Undine is able to marry a humanoid it gains an immortal human soul. While it has this soul in its possession any feature or spell that would determine its creature type or intentions identifies it as a humanoid and harmless.

Spellcasting. The Undine is an inate spellcaster. Its spellcasting ability is Charisma (spell save DC 15, +7 to hit with spell attacks). The Undine has following spells prepared:
At will: Shape water, Friends, Druidcraft, Minor illusion, Command
1/day: Wall of water*, Control water*, watery Sphere*, Tidal wave*, Suggestion, Charm monster, Confusion
*The Undine can use these spells at will if it is in a body of water greater than 5 feet cubed. 

Multiattack. The Undine makes two Water Jet attacks

Water Jet. Ranged Weapon Attack: +6 to hit, range 60/120 ft., 1 target. Hit: 10 (2d6 + 3) piercing damage. A creature on the other side of the target in a straight line, within 60 feet of the Undine takes 5 piercing damage as the water jet pierces through the first target.