Those are pre-made sidekicks found in Dragon of Icespire Peak and its follow-up adventures. These were not added in Tasha's, but follow the rules presented.

I saw where D&D Beyond is working on support for Sidekicks, but I'm wondering what this might look like. Does anyone know?

I'm hoping it's something like a scaled-down version of the character creator. The Expert, Spellcaster, Warrior categories are handy, but, honestly, with a resource like DDB I might as well be able to give them a class and subclass, or mix and match skills and spells according to what I need. For example, where does my boy-genius gnome artificer fit in? Is he an Expert or a Spellcaster? Neither of which fully fits the functions I have in mind for him. How would DDB give me the functionality to create a simple stat block (aside from the free-form Monster Homebrew, which I ain't got time for) with enough support to flesh them out as an NPC? 

As a newish DM I feel like this is an opportunity for DDB to add support for NPCs in general and party interactions with them, such as a Loyalty Tracker using the option laid out in the DMG. Combat tracking gets all the attention (understandably), but there are a lot of tables that emphasize Social interactions, but there's so little (i.e. none) for Social. As it stands, sure I can pen/paper this stuff, but I increasingly find my organization at table divided between my One-Note prep notes, DDB for combat and PCs, and notebook for in-session notes. 

I also feel like control over a Sidekick needs to be flexible according to individual tables, so maybe there's levels of access between DMs and PCs. If my Party runs them in combat that's fine, but I'm not sure they're responsible enough to treat them like individuals and not just slaves and cannon-fodder--this is basically a question of who RPs the Sidekick and how is that reflected in DDB. 

Quote from stercules >>

What are those "experts," "spell casters," and "warriors" that you can add as a sidekick? I think you can add them from lvl 1 to 12? I had not seen those until Tasha's was released...

Those are located under "extras" on your Dndbeyond chrachter sheet. They are the premade sidekicks that are listed in specific adventures like "The Dragon of Icespire Peak." 

The sidekick rules in Tasha's are a set of guidelines that let you turn an NPC or Monster into a sidekick. Having access to Tasha's doesn't automatically grant you access to the premade sidekicks. 

So, I took to the Extras tab on a character sheet, selected "Manage Extras" > Add an Extra > Sidekick and found an option to check the box "Filter using sidekick rules". Now, if I leave it checked, nothing populates in the list, even if I search. If uncheck the box, then a list of creatures populates that I can add... how is this supposed to work because I can't make heads or tails of it (pun intended).

Quote from TherynHellvalor >>

So, I took to the Extras tab on a character sheet, selected "Manage Extras" > Add an Extra > Sidekick and found an option to check the box "Filter using sidekick rules". Now, if I leave it checked, nothing populates in the list, even if I search. If uncheck the box, then a list of creatures populates that I can add... how is this supposed to work because I can't make heads or tails of it (pun intended).

 My guess is that you don’t own Dragon of IceSpire peak here on DDB.  If you did, the sidekicks from that adventure should show up for you when you have “filter using sidekick rules” on.  But, anyone who doesn’t own it wont’ see anything when that is checked.  It is currently understandably confusing because with the Essentials Kit/Dragon of IceSpire peak released, there were not full official rules for sidekicks:  there had been a UA (unearthed arcana/playtest content), but nothing official.  The sidekicks with the Essentials Kit didn’t exactly provide a rule set, they were pre-generated characters with “level up” notes through 4th or 5th level.  When the extra adventures related to the Essentials Kit were released here (and only here), those notes were expanded.

Now, however we have official side kick rules in their full form in Tasha’s.  The nature of those rules will require a whole new system.  So the sidekick reference under extras is  to the Essentials Kit type of sidekick, not Tasha’s sidekicks.  If you are wanting Tasha’s sidekicks, for now the closest you can come is to homebrew a monster with the right stats and add it as “miscellaneous” in the extras section.

The variant class features and racial origins stuff seems to have been the primary focus, so it seems reasonable to give them a pass on not having the sidekick stuff worked out beforehand.

i definitely am waiting for it too though.

Ya , no. First thing I bought on dndbeyond was dragon of ice spire peak as it’s the campaign I’m currently running. So, that doesn’t explain why I see nothing when I check that box. 

but anyhow, I built one using home brew. Still an odd behaviour. 

Quote from TherynHellvalor >>

Ya , no. First thing I bought on dndbeyond was dragon of ice spire peak as it’s the campaign I’m currently running. So, that doesn’t explain why I see nothing when I check that box. 

but anyhow, I built one using home brew. Still an odd behaviour. 

 Hunh, that is strange. WHen I try on a test character, I can see a lot of the sidekicks.  You might check to see if your entitlements are in sync:  click on your user name in the upper right corner of this page,  click on Account, then the Marketplace tab.  Does it say your entitlements need syncing?  IF so, click on the sync button.

yep, it sure looks like they are there now. it looks like they sidestepped the issue of managing the sidekick's class and level by simply making a separate sidekick for all 3 classes and 12 levels. that means, whenever you advance the sidekicks level, you'll need to remove it and add a new one, and redo all the customizations. a little hacky maybe, but it gets the job done without them having to build an entire new kind of editor.  surprised they didn't make some kind of announcement for this, maybe it's in test.

Quote from brectangle >>

yep, it sure looks like they are there now. it looks like they sidestepped the issue of managing the sidekick's class and level by simply making a separate sidekick for all 3 classes and 12 levels. that means, whenever you advance the sidekicks level, you'll need to remove it and add a new one, and redo all the customizations. a little hacky maybe, but it gets the job done without them having to build an entire new kind of editor.  surprised they didn't make some kind of announcement for this, maybe it's in test.

 Those are the Icespire Peak sidekicks, not Tashas sidekicks. 

In the view I'm getting, I'm see each possible iteration of the possible sidekick classes and levels, not individual named sidekicks from Icespire. I suspect this is just a staggered rollout of the new feature.

OK. Since Icespire sidekicks are named, and the level advancement stops at 6th, I thought these unnamed sidekicks that go up to level 12 were something different. They were not available to me before, they just randomly appeared at some point, and I've had access to Icespire Peak since sometime last summer.

Quote from brectangle >>

OK. Since Icespire sidekicks are named, and the level advancement stops at 6th, I thought these unnamed sidekicks that go up to level 12 were something different. They were not available to me before, they just randomly appeared at some point, and I've had access to Icespire Peak since sometime last summer.

 they would have appeared when the follow-up adventures to Ice Spire Peak were released here, which was later that Icespire Peak's release.  But everyone who bough the physical essentials kit got those follow up adventures for free once they were released, if they had redeemed the code for access to the adventure here.

Huh. Well I recall no such access code. 🤔

Quote from TherynHellvalor >>

Huh. Well I recall no such access code. 🤔

 Did you buy the physical essentials kit?  I ask because you mentioned buying Icespire Peak here on DDB.  

I bought the physical set, but it turns out I have the digital version because it’s shared with me through a campaign I’m in with another dm whose shared it. Which would explain probably how I don’t see the sidekicks properly. But I still don’t recall a code coming with the kit for the digital version...

Quote from TherynHellvalor >>

I bought the physical set, but it turns out I have the digital version because it’s shared with me through a campaign I’m in with another dm whose shared it. Which would explain probably how I don’t see the sidekicks properly. But I still don’t recall a code coming with the kit for the digital version...

 There should have been a sheet of paper (maybe cardstock?? I don't have physical version) in the box with QR codes (and printed codes) to redeem a free copy of the adventure here and to get 50% off the PHB.  Maybe you missed it?  Or maybe there was a packing error or someone messed with the contents?

But yes, if Icespire peak is being shared with you, the sidekicks would only show up for your characters that are in that campaign.

Thank you thank you thank you! I went back and found it. Guess I rolled a 1 on my perception when I examined the kit's contents initially. But, what sucks is that I bought the PHB at full price without the discount it seems :/

Contemplating adding a sidekick to a campaign of mine before new DDB features are complete for sidekicks.  I am thinking about adding a homebrew sub-class to the closest associated class (i.e. Class - Rogue / Homebrew Subclass: Expert).  This seems like the best way to rebuild the automation around leveling through lvl 20 from the Tasha tables.

Also, by not connecting the character to a specific party member, if that player doesn't show for a session the sidekick will be easier to continue to play I am thinking.

Thoughts?