Did you ever notice that every character might drop after defeating of specific enemy (always the same at this current level)? Please, please, PLEASE, make table of enemies behind which there are rare drop-allies! Especially for Chapter 4...
For example, in some level we need to defeat rightmost Cybermen in wave 4/6, and then might drop our new ally. But this ally NEVER drop after defeating of any another Cybermen at this level.
So if somebody will make this statistic it will help a lot)