Brainy is a quiet, kind of creepy kid who stalks Helga around and often randomly showing up in the least expected places whenever Helga is present, the most notable occasions being "Haunted Train" (in which, comically, he is the only character to possibly witness the annual occurrence of the real haunted train unless it was returning from the steel mill) and "Wheezin' Ed". This leads to one of the other kids asking him "What are you doing here?" to which he responds either with "Um...I dunno" or "Um...something?" Other than that, Brainy rarely speaks. Craig Bartlett has stated that he may have some supernatural quality, allowing him to move freely.
In the book "Arnold's E-Files", it's revealed that Brainy is quite verbose and poetic, like Helga, and that he usually doesn't speak since his wheezing is due to allergies.
Brainy's family only appears in the episode Parents Day.
He is usually seen wearing a gold-colored shirt with a white collar and brown pants underneath. He has spiky hair, wears round glasses and is constantly smiling eerily. By far, the most distinguishing traits of Brainy are his trademark wheezing and deep voice. In The Jungle Movie, Brainy appears to have started growing a mustache.
Brainy usually gets along okay with his classmates at P.S. 118, though he doesn't interact with a lot of them. Arnold once saved him and Eugene from bullies in "Mugged", and he helped Harold in "World Records". There's one time when Arnold got mad at Brainy in Wheezin' Ed because Brainy made an unexpected visit to the cave and scared everyone inside. Other than that, Brainy usually plays sports with the other P.S. 118 students.
Brainy has a crush on Helga (as evidenced by the gags in "Helga's Love Potion", "Helga on the Couch", and Hey Arnold! The Jungle Movie), which is the main reason why he stalks her. His crush on her is also a major part of the story of "Arnold's E-Files". According to Craig Bartlett, Brainy is aware of Helga's love for Arnold, and despite his feelings for her, he understands that Helga will be happier with Arnold.
- In "Operation Ruthless" Helga travels through the house of mirrors to chase Arnold and Ruth. Soon after reaching a large mirror room, wheezing is heard and Brainy's face appears in the rooms many mirrors. Helga closely listens to the survey where the sound is coming from, cracking her knuckles to get ready, and after several seconds, she positions herself in the correct spot and hits Brainy in the nose.
- In "Haunted Train", when the lights on the train begin to flicker Helga complains that she can feel someone breathing down her neck. Each time the lights flicker on, a long black shadow is seen growing closer to Helga. Eventually when the lights go on the group discovers Brainy. Helga then throws Brainy out the train door into a muddy ditch. He is later seen sitting on a fence right before dawn when the train goes by implying that he didn't go home after the ditch.
- In "Wheezin' Ed", Helga hears wheezing coming from the end of the tunnel that Arnold and she are walking in. She screams and the rest of the kids run to their location. Phoebe shines her flashlight down the tunnel and reveals Brainy. Arnold asks Brainy "What are you doing here?" Brainy replies "Um... Something."
- In "Helga's Love Potion", after Helga falls "out of love" with Arnold and acts normally, she doesn't hit him, causing him to start shaking and hit himself. After Helga does back to normal, she kisses Brainy on the face, making him collapse.
- In "Helga on the Couch" he is the original reason Helga has to go for her therapy session. Afterward, he shows up while she is having one of her usual soliloquies, presenting her a plastic ring as if to propose. Helga exasperatedly asks how he managed to track her down to that specific alley, then tells him she won't hit him today, patting him briefly on the cheek and making him give her a besotted grin.
- In Hey Arnold! The Jungle Movie, Brainy appears behind Helga in Lasombra's lair. However, he presents Helga with her locket, which she threw into the river earlier. Helga says "I could hit you!" and kisses him to thank him. Brainy then plays a pan flute in joy.
- See Brainy/Gallery
- Interestingly, out of all the main students that are generally present in Arnold's class, Brainy is one of three students who have never had an episode based around a story or problem of their own. (The others being Joey and Sheena.) He is also one of the few who have never had their last name revealed. (Sid, Sheena, Nadine.)
- Brainy may, in fact, possess skills as a DJ as he is seen on the tables in "Cool Party".
- In The Jungle Movie, he is seen playing the pan flutes, though a little discordantly.
- Brainy is the first student to break the student-teacher prank boundary on April Fool's Day, as he is the one who pushes Principal Wartz into the pool at the dance, snickering after he performed the act.
- In World Records, Brainy assists Harold in achieving their own world record for the longest mechanical pony ride by "getting the quarters from the very nice people."
- His hair can be likened to Bart Simpson's hair, both in its spiky shape form and that the skin tone is remarkably similar. Coincidentally enough, Hey Arnold! creator, Craig Bartlett is married to Lisa Groening, the sister of Simpsons creator, Matt Groening.
- Brainy is the only one of Arnold's classmates not to be portrayed by a child actor.
- His name (or nickname) was intended to be ironic, as he often appears by Helga at the worst possible time, gets punched (or injured otherwise), and still continues to do so.