ROBERT & GENE
"Ours was a complicated relationship."
Deceased. Friends, while Gene was alive. And then Robert killed him, after making his life miserable. Gene asked him to do it. "Complicated" is right; Robert, for a long time, chalked up what happened to it being "his nature," and admitted that not a day went by that he didn't think of Gene and that he wasn't sure whether he regretted what he did or not. This was Robert's closest relationship in many ways, both ways they chose and ways they didn't, and over time it's affected him deeply.
ROBERT & RINGLEY
"He's not great at completing non-food-related tasks. I just wanted him out of our hair."
Vampire. Friends-ish. Robert and Ringley, along with Shirley and Ringley's "pet" Banjo, have spent at least several years and possibly a lot longer doing odd jobs for Doctor Fugue (emphasis on "odd"). They know one another pretty well and are fully comfortable with one another, but that doesn't mean they have the best relationship; Robert obviously doesn't have much respect for Ringley and mostly just figures out ways to maneuver around him. He still has no idea Robert is a monster.
ROBERT & FUGUE
"He's also not a very good doctor, I wouldn't trust him to so much as touch that arm."
Ghost. Not quite friends but on good terms? As said before, Robert, Shirley, and Ringley have been doing odd jobs for Doctor Fugue for at least a few years, possibly quite a bit longer. They know one another well enough and are comfortable enough, and Robert even took Mona to Fugue's when there was trouble, but he also obviously doesn't have too much respect for the doctor's knowledge or skills (rightfully so). Fugue, too, still has no idea Robert is a monster; in return, Robert actually has no idea at the moment that Fugue sent the entire group on a wild goose chase.
ROBERT & CHARON
"Just take all the fun out of my job, why don't you?"
Strangers? Acquaintances? The Ferryman who takes souls across the river Styx. It's unclear whether Robert ever met Charon, through Fugue, before dying, but they may have met after. IDK, not much goes here, but I think they could get along pretty well; their personalities seem like they might mesh. Could potentially be friends if Charon were open to friends. Also, there's the question of whether monsters have an afterlife at all; if so, that could make for potentially fun interactions (or at least one interaction) given canon events...
ROBERT & OTHERS
Canon...
Robert has only short, slight interactions with other characters in the comic. He seems to step forward to be the front-facing party member whenever they have to interact with adults; he also seems to know Ringley's dad at least a bit. He seems to feel no attachment whatsoever to other monsters. It could be interesting to play him off any of the other characters from Book 1, or any at all from Book 2, whether they've met before or not; hmu and we could come up with some ideas.
ROBERT & SHIRLEY
"If I remember correctly, weren't you supposed to be the one to end the world?"
Ghoul. Friends. Robert and Shirley, along with Ringley and his "pet" Banjo, have spent at least several years and possibly a lot longer doing odd jobs for Doctor Fugue (emphasis on "odd"). They know one another pretty well and are fully comfortable with one another, and got to know each other better and grew closer after canon events. Outside of Shirley's distractability with her phone, Robert obviously respects her, and he seems to enjoy her company most out of the original group.
ROBERT & BANJO
"Siiiigh."
Were-possum. Not really friends. While Robert, Shirley, and Ringley do jobs for Doctor Fugue, Banjo just kinda goes along with them; Ringley claims the possum is his "pet" but has no idea that Banjo is a were-possum. It's hard to say if literally anyone else in the group knows either (though Mona found out). Banjo doesn't know they are one either, and doesn't realize the "dreams" they have while in were-possum form are real. Robert mostly seems to ignore them and not want them around.
ROBERT & MONA
"You're not alone. You'll be fine."
Friends. Robert's second most complicated relationship! Robert did help out Mona a bit even from the start, but seemed mostly cold emotionally towards her and disinterested in whether she'd succeed (or survive) for a while; after a bit he seemed to take the ten-year-old more and more under his wing, however, to the point of going out of his way to help her, actually opening up to her, and in the end, outright sacrificing himself to give her a fighting chance. Only one immortal monster can kill another, after all. ...Something strange might be going on there.
ROBERT & PHAGOCYTE
"There's nothing I want more than to see the Phagocyte's bloodline run dry."
There is no love lost between Robert and the Phagocyte. That man is, or was, the barrier between the Shadow Dimension and the human world, keeping the monsters out and killing any who tried to get through; he is why Robert was pretending to be a cursed doll rather than a monster. He was also secretly going around trying to wipe out undead! But according to Robert, despite all his instincts wanting the Phagocyte dead, humans need the Phagocyte, and monsters need humans...
ROBERT & CROSS-CANON
...and not.
Until Mona, the actual 10-year-old child who has no business getting caught up in all this, shows up, Robert is obviously the "straight man" of the group -- and even after she does they seem to trade the role back and forth -- so anyone who would play in a fun way off that could be great. I am also into the idea of playing Robert both off of other nonhumans and weirdos, and off of normal people who might assume he's a kid in a costume and/or be unsettled by him. Basically any cross-canon is cool with me if you think our characters could bounce off each other in a fun way!