<?php
add_filter( 'rwmb_meta_boxes', function( $meta_boxes ) {
    $post_id = isset( $_GET['post'] ) ? $_GET['post'] : null;

    $characters = [];
    if ( $post_id ) {
        // Get current author.
        $author = get_post_meta( $post_id, 'author', true );

        // Get all characters
        $query = new WP_Query( [
            'post_type' => 'character',
            'posts_per_page' => -1,
            'meta_key' => 'author',
            'meta_value' => $author,
            'fields' => 'ids',
        ] );
        $characters = $query->posts;
    }

    $meta_boxes[] = [
        'title' => 'Demo',
        'post_type' => 'book',
        'fields' => [
            [
                'id' => 'characters',
                'type' => 'post',
                'post_type' => 'character',
                'query_args' => [
                    'post__in' => $characters,
                ],
            ]
        ],
    ];
    return $meta_boxes;
} );