Shelby is a fun girl but is prone to lose her temper as shown by her instantly kicking Spencer in the face after he accidentally slapped her. At the age of fifteen, she became the youngest female fighter in the world to claim the CFC champion title. She is also called the Pink Bull as her fighting name. She has one known family member, a grandmother named Edith. The calmness between each Shelby and the iCarly gang lasted on an in-and-out method. By her own admission, she despises raisin bread toast.
Shelby Marx is first introduced in the CFC championship with everyone, bar Carly, having heard of her and cheering her on. Despite the 11 year age gap, Shelby pins down Maya Feckler in under a minute, becoming the CFC champ.
When Carly jokingly challenged Shelby to a fight, she agreed to do an exhibition fight, merely for publicity and philanthropy; it promotes the webshow and the money earned from the fight would go to the charity at the urging of her manager. Before the match, Shelby explained that she would not hurt Carly, and that the match would be "just for fun." However, when Carly was accused of pushing her grandmother down, Shelby was ready to seriously hurt her. Carly later visits Shelby at her gym and tells her what happened at the press conference with Shelby apologizes for threatening Carly and they rescheduled the fight. Later on, Shelby is approached by Nevel Papperman who created a fake video to make Shelby go hard on Carly, enraging her. At the Seattle Supper center, Carly doesn't understand what's going on and holds onto Shelby's leg for the rest of the fight. After the fight, Shelby demanded the truth to find out why Carly planned to tackle her grandmother and is smacked by Spencer due to his spasm prompting Shelby to kick him in the head in retaliation. When Shelby reveals Nevel's involvement, Sam and Carly force a confession out of him with Shelby joining the two in beating him up. After this, Shelby becomes firm friends with the trio, guest starring on iCarly and doing a spit take segment for the show.
- During iParty with Victorious, Victoria Justice tweeted "I must say, #ShelbyMarx was one of the coolest characters I've played thus far. Thanks for creating her @danwarp! Loved every second of it.". Dan Schneider responded to this tweet with "@VictoriaJustice well you played "Shelby" great! Maybe she should return and fight "Sam" Hmmmm."
- The actress who portrayed her, Victoria Justice, also portrayed Tori Vega in iParty With Victorious, which is lampshaded by Sam and Freddie noting the resemblance between the two girls.