Frankenstein is a very loyal person, especially toward his master, but can become quite serious and calculative when the situation calls for it. Although Frankenstein has shown a sadistic side during his recurrent battles, like Rai, he is also compassionate towards human beings, especially towards the students in his school. Before he met Rai and the other clan leaders, he shows enmity and hostility towards the Nobles while displaying compassion and empathy for the humans who were frequently being attacked by mutants whom were contracted to the Nobles. He accuses the Nobles of being corrupt, but gradually changes his views.
His empathy for humans and human lives is known to be so great that he became a scientist of his calibre merely by experimenting on himself. He has never used humans in experiments.
He is seen on multiple occasions to be extremely organized and thus, he loathes messiness (especially in his house) this trait is almost to the point of an obsession. However, it's unclear whether the compassion for cleanliness is due to a personal preference of being neat, or because his devotion to Rai is so strong that even a single crumb or speck of dirt in Rai's presence is unbearable to him.
He has been noted to be reckless, exceedingly dangerous and bloodthirsty when he uses too much of his power, to the extent that even M-21 has made a point of it during a battle Frankenstein was in.
He has the ingrained nature of a scientist with a passion for experimentation. In the past, he used to do a lot of research work, which sometimes caused Rai's displeasure. Frankenstein's methods forced Rai to order him to seal his powers before Rai went to sleep. Now Frankenstein does his experiments with the consent of his subjects, although he sometimes indirectly forces them to. In contrast to the Union, Frankenstein treats his subjects well and considers them under his protection. The change in his ways has caused Rai to remove the seal on his powers. However, he is only supposed to use his full powers when given permission by Rai, as he was very upset in his battle with Takeo that he was disobeying a direct order from his master when he decided to use his full power. Rai usually withholds permission unless the opponent is extremely strong and the seal would handicap Frankenstein to the point where he might be harmed, like with The 12th Elder. He also takes extreme pride in his work such that after hearing of the incident with Na Yonsu, he tells M-21 and the others that losing, even intentionally, would offend him as he is the one looking after their bodies.[1] This is conveyed with a dark intensity disturbing enough to get them sweating.
Now, Frankenstein fights not only for his master but also for everyone who is under his protection. Also, Frankenstein shows concern for his master when Rai uses too much power or removes the seal that restrains the immense power of his "Soul Weapon". This worry is due to the opinion previously stated by Frankenstein that Rai's body hasn't recovered even though he went an extensive hibernation. Frankenstein worries a lot about his Master. He trains very hard to become stronger to prevent Rai from using his power.
Frankenstein is noted to be the character who utilizes his power to the highest extent possible by Gejutel.