Hi,
The variant selection mechanism works by replacing the innerHTML of the hikashop_product_description_main div with the variant's description content. So it's not just a CSS display toggle. That means a pure CSS solution won't work here.
The simplest approach is to create a view override of the show_default.php template and add a second description div with a different ID, so the variant swap logic won't touch it. You can do this from the backend via Display > Views and add, right above or below the existing description block, something like:
<!-- FIXED MAIN DESCRIPTION -->
<div class="hikashop_product_main_description_fixed"><?php
$mainProduct = !empty($this->element->main) ? $this->element->main : $this->element;
echo JHTML::_('content.prepare', preg_replace('#<hr *id="system-readmore" */>#i', '', $mainProduct->product_description));
?></div>
<!-- EO FIXED MAIN DESCRIPTION -->Since this div doesn't use the hikashop_product_description_main ID, the JavaScript variant swap function will leave it alone. The main description will always stay visible, and the original description block below (or above) will continue to swap to show the selected variant's description as usual.
If you want the variant description block to only appear when a variant actually has its own description (and not just repeat the main description), you could also leave the variant descriptions empty and only fill them for variants that have specific extra information.