Roswaal L. Mathers

Roswaal L. Mathers (ロズワール・L・メイザース, Rozuwāru L Meizāsu) is one of the secondary characters of Isekai Quartet.

Subaru Natsuki
Prior to the events of Isekai Quartet, Roswaal had taken Subaru Natsuki under his wing after the boy saved Emilia's life during the insignia incident. He obliged in allowing the teenager to work as a servant in the Roswaal manor.

After ending up in the new world due to an unknown circumstance, they went from lord-and-servant relationship to teacher-and-student. Roswaal politely reminded Subaru, "It would appear that the first lesson you need to learn is that you should never judge a book by its cover."

One day, Subaru found Roswaal's announcement about field trip to the beach as "jumping the gun". Roswaal made a point, "No, silly. Youth goes by too quickly.  Poof."

After Roswaal made an announcement about field day and there was a class next door, Subaru went to check out through the window of the neighboring classroom. Roswaal reminded Subaru along with Ainz and Tanya to return to class.

During field day, Roswaal observed that Class 2 was losing and reminded to the class (especially Subaru) about the cavalry game. Later in Subaru's shock; Roswaal, Mr. Rerugen, and their other two colleagues entered the cavalry game. After a gigantic destructive machine entered the scene, Roswaal used magic on Subaru's team, "If I were you, I'd definitely start running." Despite Roswaal and the teachers lost the cavalry game to Subaru's team, the boy did not take it well that Roswaal had lied to the class of returning to their respective home worlds for winning the field day.

During science class, Subaru apologized to Roswaal when Megumin used explosion triggering the flooding of the school. Underneath Roswaal's saying "Quite the blunder, eh? Well, everybody has bad taste.  But as far as boom booms go, this one does seem particularly extreme, doesn't it?"; he somehow was aware of what Subaru and some of the students were up to.

Megumin
When Megumin volunteered for the talent show, Roswaal obliged and took the whole class out into the playing field. "Now, since we don't do boom boom in the classroom, I have a target here for your fiery pleasure," he gave her a bronze statue to blow up.

Whether it was intentional or did he turned a blind eye in science class, Roswaal did not stop Megumin from using Explosion magic as he somehow was aware of what she and some of the students were up to.

Aqua
The first punishment Roswaal carried out on Aqua was bucketing after he saw her attacking Ainz and his followers on the first day of school.

After catching her along with Kazuma, Grantz, Neumann, and Koenig in their attempt to steal from the principal; Roswaal subjected them to detention after school.

During test day, Roswaal caught Aqua cheating and ordered her out of the classroom before she received an automatic zero.