Its a Bank with good privacy:
Discussion
Is Cashu an alternative to Lightning or works with Lightning?
Cashu works on top of Lightning. Its a implementation of chaumian ecash.
You can build a custodian with very good privacy as the Mint (the Bank) doesn't see users balances and transactions.
You can also do really neat stuff with ecash because it's literally a blob of data. E.g. attaching it to other data like putting it in an http header.
Cashu works with Lightning. It's a better way of building custodial wallets. Btw, I made LightningTipBot, I saw you mention it.