Looking at the dots, I would say the two above Nightmare are Lizardman and Astaroth, respectively, because Ivy, Lizardman, and Astaroth made up the group known as "Schwarzstrom", all in accordance with Nightmare. Astaroth is the one on the bottom, because that line is the only one that also lines up with Maxi.
The one underneath Sophitia is most assuredly Tira, since she has behind the scenes machinations with those same lines, and all of these have a coinciding shared dot under 1585. Tira's line also goes on to interact with Sophitia, which should be rather apparent why.
So the one underneath Ivy seems to relate to her, and be the bridge that leads to Siegfried regaining his sanity. Is it possible that maybe, just maybe, Cervantes might could turn around and be a good guy in this? Removal of the Soul Edge taint, Nightmare defeated, he reconciles with his daughter, aids in her research, and helps restore Siegfried in the same way that he has found himself?