Koi Sirens are considered fully mature once they reach the age of 20, where they cease aging. they begin to age again over a duration of 50 years if they don't make any meaningful emotional attachments to another creature.

You can breath air and water.

+2 to Charisma Score, +1 to Constitution

Koi Siren are naturally beautiful and possess a human's top half with a koi fish's lower half. Gills protrude past this split to line their upper shoulders. Glistening sparkles radiate as koi siren float, and gooey finger webbing forms as they swim.

You know the friends cantrip. Once you reach 3rd level, you can cast charm person with this trait; you regain the ability to cast it when you finish a long rest. Charisma is your spellcasting ability for these spells.

You possess a fly speed of 20 feet. If attempting to move vertically with this speed, you can only do so up to 10 ft. into the air, and only if an object or structure below you could normally support your weight. These objects are also affected by your weight as if you were directly on top of them.

Your creature type as monstrosity. You can speak, read, and write common, celestial, and aquan.

One creature up to 30 feet away who hears the verbal component of an enchantment spell you cast has disadvantage on their first saving throw to resist the magic. You can use this trait a number of times equal to your proficiency bonus and regain expended uses when you finish a long rest.

You have Blindsight of 60 ft. while underwater that can only be used to see things in the same body of water as you.