The series Pilot has to do a lot of things. It has to establish the main character, set up the central conflict, and convince the network execs to okay the series. Usually, you can introduce all of the main characters in this process. Any of these qualify as the Second Episode Introduction. The key characteristic is that whoever is being introduced must show no signs of existence in the pilot, and must be a regular cast member from that point onwards. Examples of Second Episode Introduction include: