In Shrek, Thelonious is seen lacking intelligence as he is seen when Farquaad wants to choose a princess. He wants him to pick number 3 but he held up two fingers. He is later seen at Fiona's wedding, writing the word 'Aawww' on one of the gesture signs.
In Shrek in the Swamp Karaoke Dance Party
In the short, he sings with Gingy.
In Shrek 4-D
In the short, Thelonious kidnaps Princess Fiona. In the near end, he redeems himself by betraying Lord Farquaad's ghost and rescuing the characters from Fairy Falls, then he presumably lived in new life of Far Far Away.
He seems to be not as smart as the others of Farquaad's men. This was first shown when he tells Farquaad to choose princess number 3, but only holds up two fingers. While not very smart, he has been shown to have a sort of aggressive side, breaking a mirror with no problem. However, he does have a compassionate side when he betrayed Farquaad's ghost and gestured the crowd to 'Aawww' during Fiona's wedding.
He is tall, slender, and muscular, he has glowing green eyes, he wears a gray suit with a darker grey vest, gloves and shoes, and a black hood. His true face is never seen.