Group names by group of letters - Twig, ACF

I'm using ACF and Timber (twig) for Wordpress.

I'm trying to group names in group of letters. For example :

**A-E**
Arno
Adrieno
Arvay
Emily
Etienne

**F-J**
Fanny
Franck
Fergy

**K-O**

etc..

**0-9**

I succeeded in doing an alphabetical classification of an ACF repeater field.

$index_list = get_field('index_list_repeater');

foreach ($index_list as $key => $row) {
    $the_name[$key] = $row;
}

array_multisort($the_name, SORT_ASC, $index_list);

$context['index_list'] = $index_list;

But how can I group names by letter? How would I add a range (A-E, F-J etc.) with Twig?

{% for item in index_list|range(A,E) %}
     <p>{{ item.index_name }}</p>                           
{% endfor %}

That's easy, you just need to go through two loops. For example A-L will look like this:

{% for letter in range('a', 'l') %} 
    {% for item in index_list %}
        {% if item.index_name|first|lower == letter %}
            {{ item.index_name }}
        {% endif %}
    {% endfor %}
{% endfor %}

Update #2: For numbers it will look like:

{% set index_list = [{ index_name: 'Alex'}, { index_name: 'lola' }, { index_name: '5 Master' }, { index_name: '2 Master' } ] %}

{% for letter in range(0, 9) %} 
    {% for item in index_list %}
        {% if item.index_name|first|lower == letter|lower %}
            {{ item.index_name }}
        {% endif %}
    {% endfor %}
{% endfor %}