In the season two premiere, “Chapter 1: Madmax,” we saw a group of people exiting a building and one particular young girl who caused a tunnel to collapse, or so we thought. That girl, Kali (Linnea Berthelsen), plays a major role in “Chapter 7: The Lost Sister.” If the title doesn’t give it away, let me explain. Kali and Eleven, aka “El,” (Millie Bobby Brown) have a connection.