Rinthehecker

The two became friends early on. When Rin was experiencing sleepwalking-like episodes, kantje was her main protector. During kantje's evil armour arc, Rin tried her hardest to save the friend she once knew from the corruption. She never stopped believing that the old kantje was somewhere in there, despite all the hurt he caused her (he even took one of her lives).

During the kantje's multiversal arc, Rin was only sometimes able to accompany him, but they remained beacons of support for each other. Their feelings for each other took a romantic turn, but they only confessed shortly before kantje died.

Effy

Friends from basically day one, as close as brothers. The two opened the Strings of Luck casino together, and were partners in crime of sorts. Effy, a magical puppet, entrusted kantje with one of their wires, which whenever pulled on, calls Effy over. Effy was a great aid in the quest to find Onion.

However, at the start of the armour arc, Effy (while in a god-like state) ended up sabotaging the plan kantje had for finding Onion. This made kantje extremely resentful of them (the corrupting armour only made things worse). Effy was one of the main people trying to stop kantje, and to cure him.

Their relationship continued to have its ups and downs, but by the point where kantje sacrificed one of his lives to save Effy during the multiversal arc, their status as brothers was firmly cemented.

Winsweep

A menace of the server, he intimidated kantje a bit at the start. During the armour arc, Win discovered kantje's twisted actions, and set out to take revenge. Because kantje's main weapon were memory erasing potions, Win attacked kantje with so many of them at once, he completely erased his memory.

Sometime after kantje's memory (and corruption) was fixed, Win wanted to go on a redemption arc of his own. He tried to befriend kantje, who hated Win with a passion. During the multiversal arc, their relationship developed from frenemies, to (begrudgingly, from kantje's side) friends, to genuine friends.

Onion

His beloved daughter. kantje's main goal was getting her back, up until he succeeded in chapter 1.9. The intensity of his love for her drove him to do horrid acts in attempt of saving her.

Because Onion's biology is strange, she grew from age 6 directly to age 16. Losing Onion made kantje terrified of something happening to her again, which meant that he was a very stifling parent, unwilling to give her the freedom a teenager needs. This made Onion want to rebel, and make more of a mess than would've happened otherwise.

kantje also did not want to tell her about all the things he committed to get her back, afraid that she'd think lowly of him. When Onion inevitably found out, she was upset.

Eventually, through a lot of communication, kantje was able to let Onion adequately spread her wings. They managed to get their relationship to a fairly healthy state.

I would honestly place the fatherly relationship he has with Onion as kantje's most important one.

The Pariah

(Side note: a large part of The Pariah's characterization occurred on the @ MidwayGuardian twitter account!)

The Pariah is the guardian of the narrative of all multiverses. Starting from the multiversal arc, it was a strict supervising figure for kantje. Over time though, The Pariah softened up, and the two became friends.

However, ZolisterTV decided The Pariah was getting too bold, so he reset a lot of its memories. (The old Pariah was called Morning Star, the new one Evening Star). kantje was crushed to lose his friend, because the Pariah was cold and strict once again.

Like before, the kantje and The Pariah became friends with time. This time, The Pariah was able to keep star's memories, and learn more about what it means to be alive and free.

Other

Others examples characters important for kantje's character: Mugm&Silva, Boomie, Kaboodle, Bormethius, Vi.