- "Hey, Kate, wanna watch me play football?"
- ―Mark to Kate
Mark Sanders (born July 3, 2005; current age: 8) is a supporting character in Sibling Rivalries. He was the boyfriend of Kate McReary, but they broke up off-"screen", and Kate later starts "dating" Salvador Real.
Mark returns in Season 3 onward with a slightly larger role. Mark finds out that Kate is now dating Salvador, and admits that he still loves her, so he tries to get her back. At first Kate isn't happy to see Mark, but after finding out that Mark never broke up with her, they get back together, and Kate breaks up with Salvador.
- Sibling Rivalries Pre-Series
- Sibling Rivalries Main Series (Season 1)
- The McRearys Get Schooled! (mentioned only)
- Locker 239 (first appearance)
- San Salvador's Story (mentioned only)
- Sibling Rivalries Main Series (Season 2)
- The McRearys' Halloween Special! (appears as a Halloween decoration)
- Happy New Year! (mentioned only)
- Sibling Rivalries Main Series (Season 3)
- Sibling Rivalries Main Series (Season 4)
- Sibling Rivalries Main Series (Season 5)
- Trading Places (non-speaking cameo)
- A Christmas Story (mentioned and pictured only)
- Straight Outta Green Bay (mentioned and pictured only)
- Uncle Cameron Moves In (mentioned only)
- The Lockdown (non-speaking cameo)
- ApRiL fOoLs DaY: wIsCoNsIn StYlE!
- Sibling Rivalries Main Series (Season 6)
Total Number of Series Appearances: 10
- So far, "Locker 239" was the only physical appearance of Mark, until Season 3.
- When Mark appeared, he was always shown sweaty from playing football, but from "The Pool Date" onward he no longer appears sweaty and he isn't shown to like football anymore.
- It is unknown if Mark is on a football team, such as the Wisconsin Warriors.
- Retcon: Mark and Kate break up in "San Salvador's Story", but it is not seen. However, at the end, Kate is still seen happy and does not mention breaking up with Mark (although it may be because she accidentally mistaked fruit punch for liquor, like the other siblings did). Breaking up with Mark is not mentioned until "Happy New Year!", but in The McRearys' Halloween Special! (which takes place between "San Salvador's Story" and "Happy New Year!"), she is still seen to like Mark, although they broke up. Of course, "Happy New Year!" was written at least a month or two after the Halloween comic.
- Kate breaking up with Salvador and getting back with Mark in Season 4 was a choice by GTA Nerd, as he felt that these two were a better couple than Kate and Salvador, as they are the same age (unlike Kate and Salvador, who are eight years apart).
- His favorite singer is Ariana Grande as shown in "ApRiL fOoLs DaY: wIsCoNsIn StYlE!", although this was a non-canon comic so it is unknown if this is actually true in the real continuity; GTA Nerd confirmed it is.