Looks like there are still global and local accounts even now that its Yuku. 

I think the info you need is at

Changing your username Help! There is a group name displayed behind my name!

What is an extension, why is it there,  how did it get there and how can I get rid of it?

Every username on yuku has an extension. The two most common ones are .u and .nameofgroup.
The usernames that have .u behind them are called global profiles while the ones with the name of a group behind them are called local profiles.


About that ".u" suffix.  The only time I can recall seeing my account name as "MattSkosh.u" is when I join one of the Yuku chat rooms.