Losing one's Geo is a terrible thing and it's so much more likely if you carry it about on yourself.
-Millibelle the Banker
Millibelle the Banker is a Merchant in Hollow Knight. She provides banking services for a single fee.
Lore
Despite the lack of activity in Hallownest, Millibelle opened a bank in a recluse and safe area in Fog Canyon, next to the abandoned Queen's Station. Instead of a vault, she uses her shell to store Geo - a detail she tries to brush off.[2]
However, behind her kind and polite facade, Millibelle is a thief whose purpose of using her shell to store Geo is so she can simply run off with it easily. If she gets enough Geo and is given the chance, she abandons her position with all the stored earnings.
In-game events
Millibelle can be found in her bank at the base of Fog Canyon. For a small starting fee of 100, she provides a service to keep the Knight's Geo safe. Geo deposited in her bank is retained upon death and can be withdrawn at any time, up to a maximum of 4500.
If she holds 2550 or more and the Knight rests at a Bench or uses the Stagways, visiting the bank again reveals that Millibelle has abandoned her stand. Hitting the empty stand three times reveals that it was a 2D cutout. The bank sign also falls to the ground on the way out.
After she disappears from Fog Canyon, Millibelle the Thief can be found at the Hot Spring in the Pleasure House in the City of Tears. Here she can be bounced around harmlessly, causing Geo to fall out of her. The amount of Geo that can be reclaimed is roughly the amount of Geo deposited times 1.5 for a total of up to 6834. Note, Geo stops falling out of her if there are at least 80 different pieces of Geo that have not been picked up.
Dialogue
First Encounter
Hello there dearie. I was about to close up shop and run, till you came along. I provide a banking service if you're interested.
Losing one's Geo is a terrible thing and it's so much more likely if you carry it about on yourself.
My vault is much more secure. In all my time as a banker, I've never lost a single Geo!
Maybe you'd considered opening an account?
Refusing to open an account
I understand. It's difficult to part with hard won Geo. I'll be here if you change your mind.
Not enough Geo to open an account
Oh, hang on. You don't have enough Geo to buy an account, sadly. Once you've collected 100 Geo, come straight back and we'll get your account opened up.
Listening again after refusing
Have you reconsidered opening an account? Losing Geo is a terrible thing.
You'll be much more confident out there in the big, dangerous world, knowing it's safe with me.
Refusing again
Still not interested? You're far too hesitant. Make bold decisions and follow them through, that's the trick to seeing your Geo grow.
Opening an account
You're a smart one. Next time some dreadful beastie gets the best of you, you'll be glad to know your Geo's safe within my shel- errr...my vault.
Now let's do some banking.
Dream Nailed
...When next to return to that delightful spa?...The waters so ease the tension in my shell...
Greeting
Hello dearie! Keen to do some banking?
Depositing
Well done dearie! I'll keep your Geo safe.
Withdrawing
Thanks for the withdrawal. Don't go spending it all at once!
Cancelling
Changed your mind? Well, come back any time.
Farewell
Goodbye then. Stay safe.
First encounter
Oh...A-ah, hello again d-dearie. Fancy running into you he-here.
Is it your Geo you're after? We-ll you see, it's ahhhhh. It's a sad thing, but...It's gone. All spent.
The cost of running a bank can be high, so I had to charge a somewhat large service fee to your account.
Perfectly legitimate, of course. Just part of the whole banking deal...ah ha ha...ha.
But...ahh..thank you so much for visiting me, and run along now won't you. If you allow me some privacy, I'll be done here soon. Then you can have the spa all to yourself.
...No h-hard feelings, okay?
Greeting
Wah? Oh... you- you're back. I just thought, when you left, that was the last I'd... ahh... see of you.
It's just... ahh... your Geo issue. You should know, it was a big stress on me as well. Such a stress in fact, that I've left the banking game for good.
No Geo left
I--I've nothing left...I ahhh, I suppose, ahh, you might be on your way now then?
Y-y-yes, I think that would be b-best...
...I think I might j-j-just rest right here a little while. Yes I think that would be best.
Dream Nailed
...Oohh no... Leave me be, creepy little thing...
Location
Millibelle's bank is found near the Queen's Station in Fog Canyon but she runs away to the Pleasure House if she is visited in Fog Canyon after depositing 2550 or more and resting on a Bench or using the Stagways.
The 100 the Knight deposits when opening the bank account is not returned.
At the Hot Spring in the Pleasure House, she is called Millibelle the Thief.
She can be bounced out of the screen through the room's exit but does not change rooms. Exiting and re-entering puts her back as usual. This decreases how much Geo she can drop.
Dreamshield "tinks" off of the bank stand as if it was an enemy or damageable object.
Millibelle is one of the few NPCs who can be "hit" with the Nail without taking damage.
References
↑Official manual: "Visit her often if you fear your Geo may ever go missing."
↑Millibelle: "Next time some dreadful beastie gets the best of you, you'll be glad to know your Geo's safe within my shel- errr...my vault. Now let's do some banking."
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100, she provides a service to keep the Knight's Geo safe. Geo deposited in her bank is retained upon death and can be withdrawn at any time, up to a maximum of 
Unbreakable Greed has no effect on the amount of Geo the Knight gets back from her shell.
