Mario Kart is at heart a multiplayer game. The stated goal of the game is to beat other racers, often player-controlled. The single player mode, created for the expectation that a single-player mode is nonetheless necessary, is identical except it substitutes computer-controlled racers in place of the player-controlled ones.
As discussed in lecture and in other blog posts, the presence of other players significantly alters the modes of play. While there certainly are single player components a play-centric approach could analyze (racing backwards, trying to go out-of-bounds, etc.), multiplayer mode specifically gives rise to competitive or cooperative targeting (among other actions): trying to beat other human racers, or trying to help them instead.
However, what is notable about Mario Kart is that human-racers are identical to computer-racers mechanically (outside of variable skill... or cheating). Hence, there is a categorization between oneself, other human actors, and the computer racers, and it seems that competitive or cooperative targeting can solely arise from multiple people.
Despite this divide, there is a blurring of the divide between human and computer racers. In every race or series of races, there is always a skill gap between the computer racers, in some sense mirroring the on between people in real life. Hence, a common phenomenon (in my experience) is to elevate the more skilled AI, or at least the AI equally skilled with the player, to a near human status. One can compete specifically against Baby Mario (or whichever NPC is best), anthropomorphizing them based on their actions (such as always hitting you, the player ,with a red shell). Else, one can deliberately help a hapless CPU by harming their direct competitors rather than race for oneself.
It is important to make a distinction between these player generated narratives with the background of the characters in the lore. While Luigi may be a mild mannered overshadowed younger brother in official Nintendo sources, he becomes a single-minded antagonist in the player (meme) lore. Hence, it is not a design impetus to create some sort of character identification as in a story-based game, but rather a more fully organic play-centric experience.
I find it fascinating that even a potentially single player experience can give rise to play-centric modes that are traditionally found only in multiplayer gaming. This is not to say that the extent to which this occurs can match the diversity of (the CPU will never race backwards without mods). However, it remains that at least some blurring does occur, and we may question whether or why certain (multiplayer specific) genres can duplicate these experiences or not. I, for one, will probably never when I player against bots in League of Legends or even to a lesser extent Super Smash Bros, for lack of human skill or playstyle, that I might have for Mario Kart (or the opposite occurring, mistaking people for computers in Moirai). I am curious to the possibilities of computer AI to duplicate multiplayer experiences, or more broadly to how designer response to play-centric modes may give rise to future formalistic incorporation of such play.