Per chat, disappearing can be off, 24 hours, 7 days, or 90 days. You can also pick a custom time. A single message can carry its own timer, separate from the chat setting. A small clock on the bubble shows when it goes away.
A photo or video can be view-once. Opening it marks it viewed. Kissend does not warn you today if the phone takes a screenshot of that photo.
Delete is a different action
Delete for me hides messages on your side. Delete for everyone removes your own message for the people in the chat. A group owner can delete someone else's message for everyone. There is a short undo after a delete.
Clear the chat for yourself, or for both people. Clearing for both asks you to type the chat name, so it is harder to do by mistake. An export of the chat before clearing is not available.