Arnold first appears in episode 21, as him and his wife Vitoria are talking to Giles.
They are said to be cool parents, as they allow their children do whatever they want as long as it's not bad for them, such as allowing Armin to stay up all night to play video games.
Arnold and Victoria are the twins' adoptive parents, hence the boys don't really resemble them by looks. As they are really good parents and the twins love and appreciate them very much, this topic isn't the slightest bit sensitive to them.
While Alexy is very much like Victoria, Armin has a personality similar to Arnold's.
Arnold has blonde hair and a beard, and green eyes.
He's wearing a dark red shirt with a symbol on it, and grey pants. It is mentioned by the twins that Alexy picked his father's clothes for the school's open house day, which might mean that Arnold himself doesn't like shopping or fashion, much like Armin.
Arnold is calm and laid-back. His personality is a lot like Armin's.
As a parent, he is also easygoing, and mostly anything but strict with the twins.
- His name is originally spelled Arnaud in French.