Milk

MIlk is referenced in all Karlson games. Karlson is addicted to milk and (in the 3D versions of Karlson) is tasked with locating and taking various cartons of milk. Also, in Milkman Karlson, it heals Karlson, which is super useful.

Karlson 2D
Milk isn't given much thought, as the entire point of the game is to only kill every enemy in a level. It makes an appearance on Karlson's counter in a random building, and is mentioned in the Itch.io page.

Itch.io 3D Version
This version gives the player one single goal: capture the milk. It is always placed away from the player and the player can do whatever necessary to get it, such as blow themselves up into the air, kill enemies, and do parkour. It is never explained why Karlson wants milk so bad, as it does not do anything but win the level. It marks nothing other than that Karlson enjoys milk, and it is how you progress through this version of the game.

Steam Version
Unlike the 2 other versions of Karlson, the Steam version tells the player his mission in a short video on a TV screen in a small room at the very beginning. Unlike other versions, Karlson can take more than one hit, so he must drink milk to restore his health after his robot-fighting antics. When he drinks a carton of milk, it signifies the progression. Progression in the Steam version is also more seamless and natural, so instead of showing a win screen, he just drinks it, an explosion of white particles occurs, and he keeps going.

Milkman Karlson
In Milkman Karlson, milk heals you.