It is part of the UA and it says Whenever you gain a level, your sidekick also gains a level. It doesn’t matter how much of your recent adventures the sidekick experienced; the sidekick levels up because of a combination of the adventures it shares with you and its own training WoTC link: <a href="https://media.wizards.com/2018/dnd/downloads/UA_Sidekicks.pdf" rel="nofollow">https://media.wizards.com/2018/dnd/downloads/UA_Sidekicks.pdf</a> Normally I would not post the link (Non-Roll20 specific and such) but this is a free pdf from their content partner and it is specific to the question asked. Hopefully the UA will come to Roll20 and those who play 5e with UA content will have access to it.
