Fantage.com or Smalworlds.com
There is moviestarplanet you make avatar have your own house make friends .
Second Life.
Xivio is a free 3D Virtual World and Social Network for kids where you can do a variety of things such as create your own avatar, chat, collect rare items and, own your very own apartment! :)
It is a Harry Potter themed virtual world where you can customize your own avatar
wizard101.com and freerealms.com
Google Lively is an online virtual community provided by Google, where you can create your own avatar, virtual space, as well as chat in virtual chat rooms.
Be a virtual mom Look after your own virtual baby Play with other girls for free .Babydow.com
imvu
You cant.
Usually, you make your own avatar when you sign up, but you can buy clothes, hair, costumes, boards, etc... for you avatar.
Meez is a virtual online world where you can make your own avatar and customize it in your own way. You can chat with people and invite your friends. You use "coinz" to buy clothes, backgrounds, dances, animations. It is completely free, but there are some premieum items that are "VIP" "VIP" is a membership that offers more playing options that have to be payed for.
Yes, there are virtual worlds where you can interact with mermaids, such as “Second Life” and “IMVU.” These platforms allow you to create your own mermaid avatar, explore underwater environments, and socialize with others in the virtual world.