如何从主题扩展wordpress插件中的类？

I am trying to extend a class that is inside a plugin from my theme, but if I check the class to be extended with if(class_exists('Hello_Class')) I can see the class is not available. The plugin structure is something like this:

    class PluginCore {
        public static $info;

        function __construct() {
            add_action( 'after_setup_theme', array( $this, 'after_setup_theme' ), 12 );
        }

        function after_setup_theme() {
            // the class I want extend
            require_once '/Hello_Class.php';
        }

}
new PluginCore();

I want to extend the class "Hello_Class", I tried to get this work using hook like after_setup_theme and init, but nothing works. Is there a way to make this work.