Someone copy my github repository and created another github repository simillar to my own. That person changed my name with his codename. And didn't even put a credits.
Is that considered as Plagiarism? If no, can you explain why? And if yes, can you tell me what I should do?
It is plagiarism. Which is perfectly legal, unless it leads to fraud (like getting a job based on the brilliant code in a git repository that the person never wrote). In academia it will have massive consequences if found out.
More important, it is copyright infringement. Send a DMCA takedown notice to github, and the repository will disappear very quickly.
In some countries, Germany for example, it violates the right of authorship, which is your right to claim that you wrote the code. In these countries nobody has the right to say they wrote the code when you wrote it, not even your employer - as long as you wrote it. Note this is separate from copyright.