Mariappa has been impressed with both his attitude during games and personality outside of Vicarage Road.
He told the Watford Observer: "Richarlison has settled in really well. He's still brushing up on his English, but he's got a great character about him and he doesn't seem his age - he plays with a lot of personality.
"He's fit straight in and he's had a great start in the league, the boys love him. He puts a great shift in and that's the most important thing.
"His English isn't the best at the moment, and I'm not planning to teach him! But he's working on it and he's always smiling and he's got great attitude.
"You never know whether that chance was going to go in, but he's got great composure there for a lad so young and I'm delighted for him."