Faust is usually a very detailed and patient person who thinks scientifically unless the matter concerns his beloved Eliza. He does not tolerate interruptions while working (an example was having Eliza keep the X-Laws at bay, while he attempted to help Jeanne), and even less any criticism, insults or threats directed at Eliza.
Faust is the embodiment of dedication; he has given up everything for his wife and is willing to do anything, no matter the costs to protect and take care of her. It is this single-mindedness and selflessness that gives him his great strength in the Shaman Fight. Beyond all the mental instability, emotional suffering and somewhat gothic moods, his greatest strength comes from his pure heart; pure because he knows that he only lives to love and protect his dear Eliza.