One of my players is making a Grimalkin Wizard. She's taking the Noble background with the Retainers feature. Her plan is to make the other players think her Retainer is the wizard (human girl). She wants to use the retainer like her familiar (a cat with a human familiar). I decided to give her an Adept, but with Wizard spells (support only, no combat) rather than just a simple commoner. What do you think, plausible for a cat-like noble wizard to have a familiar/retainer that is a human Adept?

Personally, I think it's an interesting subversion on the old trope: spellcaster has a pet cat; now spellcasting cat has pet spellcaster.  I like the idea from a flavor standpoint, but I'm not sure it's the best idea mechanically.  A familiar is usually not capable of anything beyond moving/looking around, and setting up somewhere so its owner can cast spells through it.  A "familiar" capable of its own support magic may be a bit overpowered; but then that also depends on the rest of the party.  if it's a small party, or a party that's severely lacking in support magic, it might be okay and no be an issue.  But if the party is well balanced, other players may view it as unfair, since it gives the wizard a rather substantial leg-up.

If I recall correctly, the retainers from the alternate noble feature don't enter dungeons or otherwise get into danger. Now, as the DM, it's up to you whether to follow that. I say this only because the current plan appears to involve the retainer/familiar getting involved, and that may/may not create issues down the track like hp tracking and getting attacked or being caught in a trap. The idea itself sounds fun. I don't know if this is the best way to do it.

I'd want to know what the attempt actually is for the player.  Because, here's the deal.  If she is a spellcaster, and wants to play a mini-game of "keep my fellow players in the dark" then what happens the first time there is, well, any action whatsoever?  The only way I see it playing out is that she essentially becomes the retainer and roleplays her...but, she can't if this is a normal NPC, the GM would roleplay it. At that point, what is her actual character doing? 

What happens the first time there is combat.  Rulebook says retainers won't fight. So where the heck did that fireball come from?  Then you have all this effort to develop some kind of system outside of the RAW and RAI which instantly becomes moot because no one is going to be that blind for more than a session or two.

Clearly the "you're the GM, do what you want" rule applies.  But as a GM with a lot of experience, I can say certain things just aren't worth the effort, your time is better spent working on an engaging story and NPCs than doing backflips for someone to do something that is way far out of the norm.

Example:  I put a ton of time in on custom sounds with Syrinscape for a session or two.  It added a lot to the experience, but took soooo much work to get it "the way I wanted" which was, of course, perfect.  I then wound up creating various playlists for specific feelings: calm, combat, running, sailing, creepy, tense, sad, etc.  That took a long time to create as well, but now the effort is super minimal, and I feel it adds almost as much to the game with next to none of the effort.  So I'm sticking with that and focusing on my story.

By support, I mean party support. She'll be the normal familiar, but has no actual attack spells. The most combat-effective spells she has are defensive in nature. Otherwise, she's a magical retainer. Mending, Prestidigitation, Shield, and Tenser's Floating Disk at first level (but only able to cast one 1st-level spell). Shield, Invisibility, Feign Death, Leomund's Tiny Hut, etc. for spells. Nothing she can cast on others, extremely small Spell DC (her int is 13). She gains one wizard level every two levels her "owner" gains (meaning she'll only get up to 5th level spells at 9th-level). Additionally, she won't get an Arcane tradition and can't scribe spells in her spellbook (only gaining one per level of her owner).

It's primarily for flavor than for combat, but she will still be a normal familiar with the touch spells. Also, in public, she will be the eccentric noble who always has "her cat" with her as the cat takes control in the social situations. If she dies, then she can be resummoned like a familiar. On her own, though, she's a bit flighty and an airhead.

Sounds way more powerful than I would allow a familiar to be... or a retainer, or a ranger's pet, But that's me.  Sounds like you really already made up your mind on this one.

Because every spell this retainer casts, even if non-combat or defensive, is a spell slot the player doesn't have to use.

Also, even pact of the chain warlocks have to use an invocation to speak through their familiars, which is a significant investment.

I share the sentiment that this is getting overpowered. The noble's retainers are clearly intended as an RP feature that offer no in combat bonuses. Even though you say the  retainer will only have "support" spells, the retainer will help in other ways, by helping action economies, being within 5 ft for sneak attacks, being another target for attack, having spells that are currently not prepared that day by the wizard.

If you're going to go down this route, I'd have the retainer be a charlatan of a magician, a purveyor of simple stage tricks, a children's entertainment "wizard".

Hi, I'm the one with the original idea ^_^ This was for a one shot that I ultimately went with a Grimalkin barbarian (I tend not to play melee characters, which is why the input about this concept) The cat barbarian went over really well and was a huge shock for the rest of the party when she went into her first rage ^_^ It was a lot of fun. Thank you all for your input.