The WYSIWYG editor automatically encodes Greek characters to entities in the "text" mode. They can be stored in pure, non-encoded format only in the "Source" mode of the editor.
You can change this by turning this encoding off in the editor configuration:
Please find the ~\CMSAdminControls\FCKeditor\fckconfig.js file
and approx. in the line 79, please change the following property value from "true" to "false":
FCKConfig.IncludeGreekEntities = false;
Please note that you may need to clear the cache (temporary files) of the browser to apply the changes.