McDermott has revealed the 25-year-old will complete his switch to the Madejski on Monday, and is confident of adding another new face to this squad in the near future.
A fee for Mariappa has been agreed between the Royals and Watford, and all that remains is for the centre-back to complete a medical.
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"He's a character, a fantastic character. He has done really well for Watford over a period of time.
"For a young man he has played a lot of games, he's got good pace and as a character he will fit in with what we are trying to do here - as has been the case with all of the other players I have signed."
As well as Mariappa, McDermott has also secured the services of Pavel Pogrebnyak, Danny Guthrie, Nicky Shorey and Gareth McCleary, but the 51-year-old is still looking to bring in more new players.
"We have made an offer for a player which has been ongoing for a number of weeks," he added.
"We will wait and see. No names, not at the moment. He's in England and we are hopeful that we can get it done."
Tottenham striker Jermain Defoe has been one name touted as a possible newcomer, but McDermott says a deal is unlikely at present.
"Jermain Defoe is a terrific player, he's a Tottenham player and I don't think he'll be coming to Reading somehow," he continued.