As a child Guillaume appeared to be a bully as he often teased Jun/Hisao. Abraham suspects this is due to the fact that Guillaume may harbor jealousy towards Hisao's swift progress. The truth in fact is far less simple however as it is revealed that Guillaume has a crush on Hisao and teased him as a way of showing his affection. This shows that Guillaume is somewhat reserved with his true emotions and must find an alternative outlet to let others know how he feels, this is expressed when he shoves Jun to the ground after kissing him.
As a teenager Guillaume retains much of his earlier established persona as well as a more serious side. He is presented as tough even callous to a degree and committed to his mission and sees everything else as irrelavant, neglecting to even communicate with Hisao after he arrived, though he admits he wasn't sure if Hisao was who he said he was on account of using a different name. Guillaume also shows the willingness to use unorthodox methods in fighting something he refers to as "fighting dirty". He still carries the same level of affection for Jun, perhaps even more due to their time apart, and is far more openly expressive about it. He also shows a slightly more light hearted side to himself when in the presence of Hisao.
Before the Academy
Guillaume is shown tripping up Jun as the latter runs to show Abraham the result of his training by way of a target. He mocks Hisao calling him a "loser" and "kiss-ass" before goading him into a fight. The end is not shown, but Hisao later states that Guillaume did not succeed in taking the target from him.
He is later present as Abraham calls the six names of the children that would be enrolling at Morning Glories Academy. Before he departs he sees Hisao sulking. He runs towards him, even after Abraham calls after him, and kisses Hisao on the lips. He then proceeds to push him to the ground while smirking.
In the Academy
Guillaume is present when the students are preparing for Woodrun, but is not shown only referenced cryptically by Hisao. He later makes his first present day appearance by assisting Hisao in his fight with his twin by striking him, Jun, in the back of the head. He then states, "This was always your problem Hisao--you're too afraid to fight dirty". He helps his ally to his feet and reminds Hisao that he has orders as well. He also dismisses Hisao's brother as only a distraction while referring to the other students as "weak, broken and pathetic".
The two separate from the student body and embark on their own arriving in a seemingly abandoned school. Here Guillaume reveals that he was responsible for causing the faculty to vanish and hints that he can do more. He asserts that he needs Hisao as "The hour of our release draws near". He leads the Glory into a room and closes the door after Hisao tells him to call him "Jun" while at the academy. Guillaume feels that "Hisao" is much better before drawing closer. The two proceed to undress one another as they kiss. Later Guillaume is lying between Hisao's legs as the latter plays with his hair.
He hushes Hisao when a door slams. The two dress and watched as the real Jun stomps through the hallway with two other students. They listen as he insists that he could have killed Jun if Guillaume hadn't interfered. They follow the trio downstairs into the sub levels and watch as they are given a goat by a man named "Garrett". The group begin to prepare the animal for some special function with Hisao's brother stating, "A sacrifice is always demanded".
Next, he and Hisao are seen running across the grass towards the forest. He hurries Hisao explaining that his brother's intervention has forced them to improvise. He states that he doesn't have time to explain everything as Jun could possibly undoe all their work. He also says that they must warn "her". When Jun asks who he is referring to Guillaume teases Jun by wondering aloud why such a teacher's pet could not figure this out.