My cousin used weirds characters in his username like 😃 , may it cause some problem in future? Because that character is in path.
Windows 7 64-bit Ultimate
Windows NT can handle any Unicode character in file names; if anything breaks, it would only be individual poorly-written programs. In case of problems, it's possible to rename the profile directory and then adjust the registry. (The directory name and the user name do not always need to be the same.)
User names support full Unicode as well, but they aren't actually used by the system (SIDs are), so there shouldn't be any problem with such a username at all, and it can be renamed very easily. (Aside from occassions where you have to enter the username, for example, network logons... but one can copy/paste.)