For chats you don't want anyone holding your phone to see:
- Open the person's profile → Lock chat.
- Opening it needs your fingerprint or phone PIN.
- It appears in the Locked chats folder, and its notifications only say "Tellu · New message", with no name and no text.
- Hide this folder: the folder never appears. To open it, long-press the "Messages" title.
Your phone needs a screen lock (fingerprint or PIN).