Every DM has encountered a situation where the party grows attached to the gnome NPC they added to an encounter to balance out it's deadliness. Now the party wants to bring them along for the rest of the adventure. You don't want to say no, but you also don't want to handle another creature on top of the 11 hobgoblins and 16 goblins you're about to throw at them. You could allow the players to take the gnome as a sidekick, a mechanic introduced in Tasha's Cauldron of Everything, but that might skew the encounters even more in favour of the players. You still want to challenge them. So what's the workaround? You want to say yes and ensure their new ally has an impact.
We've created a new mechanic called NPSs (Non-Playable Sidekicks). These non-playable characters don't have a stat block or any resources to track, but most importantly, they still add value to an encounter without making it unbalanced.
The NPS (Non-playable Sidekick) is an additional party member designed to aid the players both inside and outside of combat without breaking the game. The NPS is primarily controlled by one of the players but can be used to assist any member of the party. They are more akin to a familiar than an additional PC or NPC. They level up alongside the players, gaining new abilities as they do.
- from the publisher's blurb