Blockstream Green Bitcoin wallet has a unique Multi-Signature model using dual private keys – one held by the user, and one by their servers.
This allows a higher security mobile Bitcoin wallet system that can enforce Two-Factor Authentication to protect your funds, while time-lock smart contracts guarantee that users always retain full control of their coins.
No documents, personal information, or KYC required. All that is needed is a single email address. For even further privacy user can choose to not use an email and exclude some functionality of Blockstream Green Wallet. You can also choose to connect over Tor for increased privacy.
Resources with more information.