Unearthed Arcana is back with an article from WotC's Jeremy Crawford and Ben Petrisor called Sidekicks! "If you’ve ever wanted an animal, a town guard, or another creature to join you on your adventures, this month’s Unearthed Arcana is for you. It gives you a straightforward way to turn a creature into your sidekick and have that companion level up with you."
Our current GM may have a fit if we introduce any sidekicks, as we have an especially large group for the next campaign. On the other hand, the majority of campaigns I've played in or DM'd have, at some point, seen a player or the party as a whole 'adopting' some creature or person, so I could definitely see myself getting some use out of these rules.
It occurs to me that they might also be a handy way of creating simplified NPCs, for when you want something a little more substantial than the Monster Manual stat block, but don't need to go through an entire class and subclass progression. You could also use them to beef up any monster or NPC if it doesn't quite measure up to the CR you're looking for.
I really like this. Its useful for creating cut down but functional characters that can scale. Having a tool which lets you build PC level creatures without having to make a bunch of choices is nice. I wish I had it when we played Curse of Straud so I could have let Ismark be useful the whole campaign. I play in a small group so this is also a good way to up the power level of the group without making the players juggle multiple fully featured characters.
I would never, ever, ever let a PC just decide to add a sidekick because they wanted one. Familiar sidekicks? *shudder*
Edit: Having thought about it for an additional 30 seconds, if I had a small group that was struggling, a sidekick familar actually sounds like an OK idea. Also, you can have a sidekick turn traitor in a way that you could never do with someones PC. That level 10 spellcaster imp familiar is a really nice ally to have until it turns out its been taking orders from a pit field this whole time.
I like it conceptually but I don't really like the notion of sidekick casters being able to cast such high level spells. I'd probably replace spell progression with a half caster, possibly adding in an arcarnum feature to give them limited access to a higher level spell slot.
In my campaign I allow pets and sidekicks to gain extra hit dice based on the PC's proficiency bonus which generally only means they can take one extra hit.
It's sort of reminiscent of 3e's NPC classes of Warrior, Expert and Adept as well as the Leadership feat that gave you a companion in addition to a bunch of low level followers. And they seem to be as powerful as PHB classes at first glance.
And I'm guessing that your Goblin Squire uses the Goblin statblock from the Monster Manual, and not the Goblin PC race stats.