At his core, Mr. Simmons is a very sweet and caring man. He believes in the best of everybody, and that everybody is special. Though he tries his best to stay positive and upbeat, even he succumbs to stress every once in a while. Because of stress he's been shown to do some things that aren't so nice, like snapping at his students, and nearly swearing in front of them.
Mr Simmons is an of-average-height Caucasian man. He's skinny overall, with a slightly protruding belly. As of The Jungle Movie he wears a light green sweater vest with a bright green bowtie, a white collared shirt with the sleeves rolled up, red-brown pants, and slightly darker red-brown dress shoes. He is balding, but the hair that he still has is blond. His eyebrows are blond as well and bushy, and his nose is upturned. He also wears an analog wrist watch.
Mr. Simmons is Arnold Shortman's (second) fourth grade teacher, who took over from Miss Slovak in the episode "New Teacher". He demonstrates a love for teaching immediately, forming the desks into a circle to encourage stronger interaction between students. He also encourages learning by teaching the students to always ask "why?". The students do not like Mr. Simmons' teaching style at first and plot a plan to break him down and to cause him to quit. After a series of pranks (which even Arnold participates in) Harold finally breaks his spirit by eating his lunch. Mr. Simmons quits teaching that afternoon.
The next day, the class expects a new teacher that they can break down. Mr. Wartz announces that he has a solution to the classes behavior problem. The solution is U.S. Army drill sergeant, Lieutenant Major Goose, a strict, child-hating taskmaster. The class soon realizes that class with Mr. Simmons wasn't as bad as they thought it was after experiencing Mr. Goose's tyranny. The class breaks Goose down the same way they did Simmons and make peace with Mr. Simmons by making a creative circle and inviting him in, making him rethink his resignation. Mr. Simmons remains their teacher for the remainder of the show.
In the episode "Arnold's Thanksgiving", it's revealed that Mr. Simmons' first name is Robert during the Thanksgiving play where a family scene that included characters whose names matched his own Thanksgiving guests. His other Thanksgiving guests, or rather those whom he is thankful for, are his mother Pearl, his friends Peter and Joy, and his Uncle Chuck.
He was also briefly made principal in "Principal Simmons", though his lax methods such as his kindness, when not backed up by Wartz' discipline, fall flat and create chaos. Wartz himself has a change of heart, becoming much kinder.
In the episode, "A Day in the Life of a Classroom", Mr. Simmons was named Teacher of the Year.
He is not to be confused with Robert.
- Mr. Simmons' unusual character design is based upon his own voice actor, Dan Butler.
- In his debut episode "New Teacher", he says his favorite sandwich is turkey, tomato and watercress.
- Mr. Simmons' rides a motorcycle with a sidecar.
- Craig Bartlett has confirmed that Mr. Simmons is gay, though this was never outright mentioned or shown to be obvious on the show, most likely to avoid the controversy of addressing homosexuality on a children's program.
- Mr. Simmons’ relationship with Peter is very vaguely “hinted at” in the dinner scene in "Arnold's Thanksgiving". Peter was merely referred to as a friend, along with Joy. Peter appears a second time briefly at the airport with Mr. Simmons in Hey Arnold! The Jungle Movie. However, Peter is not seen going to San Lorenzo with Robert and never seen again following the airport scene.
- 29b. New Teacher
- 30b. Ms. Perfect
- 31a. Monkey Business
- 32b. Best Friends
- 33b. Eugene's Pet
- 35b. Phoebe Cheats
- 36a. Helga's Love Potion
- 41a. The Aptitude Test
- 42a. Curly Snaps
- 44. Crabby Author / Rich Kid
- 48a. Arnold's Room
- 49a. Career Day
- 50b. Gerald's Tonsils
- 51a. Phoebe Takes the Fall
- 56. Arnold's Thanksgiving
- 57. Girl Trouble / School Dance
- 58. Helga's Show / The Flood
- 59. School Play
- 60. Parents Day
- 61b. Stinky's Pumpkin
- 62. Dinner for Four / Phoebe Skips
- 63b. Student Teacher
- 67a. Helga's Parrot
- 69a. It Girl
- 72. Back to School / Egg Story
- 74a. Helga Sleepwalks
- 75b. Buses, Bikes, And Subways
- 76b. Synchronized Swimming (in audience)
- 78. Helga on the Couch
- 83b. Principal Simmons
- 86b. Sid and Germs
- 89a. Suspended
- 92b. Curly's Girl (cameo)
- 93a. On the Lam
- 94a. Phoebe's Little Problem
- 95a. A Day in the Life of a Classroom
- 98. April Fool's Day
- Hey Arnold! The Jungle Movie