Richarlison joined Marco Silva's outfit during the summer transfer window from Fluminese and, despite being new to the English game, has contributed to the Hornets' strong start to the campaign.
The 20-year-old scored his second Premier League goal of the season to secure a valuable 2-1 win over Swansea City at the Liberty Stadium, the first time the Hornets have won three consecutive away games in the top division.
Defender Mariappa told Sky Sports News: "He's fitted into the group straight away and settled in really well. He's got character about him and his age doesn't seem to matter. He plays with a lot of personality and the boys really love him.
"He's had a great start, especially for a young lad, and puts a great shift in for the team. He's working on his English but he's always smiling. He's got a great attitude for a young kid and showed great composure for his goal."
Watford, who last won away at Swansea in February 1983, are now sixth in the Premier League table with 11 points from their first six games.