admin.php?page=quiniela/characters.php is coming from where in your codes well try this.
function add_quiniela_menu()
{
add_menu_page('Quiniela Game', 'Quiniela Game', 0, 'quiniela/quiniela_funcs.php', '', '');
add_submenu_page( 'quiniela/quiniela_funcs.php',
'Characters',
'Characters',
0,
'quiniela/quiniela_funcs.php?page=characters'
);
}