pixcel25 Firstly, what nothing device are you using?
When you go to contact/chat in Whatsapp and choose notifications then custom notifications do you not get the screen where you can then set a different Notification tone and glyph pattern?
Or, maybe try this - you get a notification from that person, long press the message notification in the notification area and set the specific convo as essential glyph.
If you are still not having luck, you might want to submit me this to the Nothing team. Settings > System > Feedback.
