Every mentor has one ending linked to one the factions plus one special unlockable ending that is only available if you are in the same house as your mentor. Apart from those, Randal also has one extra ending.
Randal I, III and IV: The faction endings. If I remember correctly, you get "Mark David" by siding with Saorise and "comorbid" by joining the rebellion. That would mean that you still need Andrei's ending.
Randal V: The special ending. First you have to unlock it by playing some of the other endings, then play as a Mavvar. Certain choices should now be marked red, simply pick those to get the ending.
Randal II: The ending that made me tear out my hair. I finally got it by siding with Andrei, while also making a deal with Lazarus. (I also visited him a second time in the sewers and heard his story, but I'm not sure whether this is necessary. I also made sure not to tell Andrei about him, but again, that might not be necessary.) After the final fight, you will get a scene in which you notice Lazarus sneaking up on Andrei. When you get the choice whether to interfere, let him murder Andrei.