Developer Console for a service is provided at the URL below.
The login form accepts a pair of ID & password of a developer. It is different from the ID & password of your Authlete account.
Authlete delegates developer authentication to your developer authentication callback endpoint, so any developer cannot login Developer Console until you implement the endpoint for the service.
However, as a special behavior, the login form accepts the pair of API key & API secret of the corresponding service as if it were a valid pair of ID & password of a developer. Therefore, if you are the only person who manages client applications for your service, you don't have to implement a developer authentication callback endpoint.
Anyway, you (the service owner of the service) can login Developer Console with the API key and the API secret of the service without implementing a developer authentication callback endpoint.
It is only when you want to let other developers use Developer Console that you are required to implement a developer authentication callback. Read "Developer Authentication Callback" for details.