get_comment_author( int|WP_Comment $comment_id )

function get_comment_author( $comment_ID = 0 ) {
	$comment = get_comment( $comment_ID );

	if ( empty( $comment->comment_author ) ) {
		if ( $comment->user_id && $user = get_userdata( $comment->user_id ) )
			$author = $user->display_name;
		else
			$author = __('Anonymous');
	} else {
		$author = $comment->comment_author;
	}

	/**
	 * Filters the returned comment author name.
	 *
	 * @since WP-1.5.0
	 * @since WP-4.1.0 The `$comment_ID` and `$comment` parameters were added.
	 *
	 * @param string     $author     The comment author's username.
	 * @param int        $comment_ID The comment ID.
	 * @param WP_Comment $comment    The comment object.
	 */
	return apply_filters( 'get_comment_author', $author, $comment->comment_ID, $comment );
}