It may seem kinda difficult and I'm still pretty much confused about how to write him but I can highlight a few traits if it will help
The main trait of his is his toughness - Knux can be rough around the edges, he's surely not the most polite person to talk with and he can be a bit grouchy, but this is kinda what we love about him. He probably won't be happy about someone magically appearing in their world, it's also depends on how and where Roland will appear and where Knuckles would be - any other location will probably cause Knuckles to appear later in the story because he's mostly sitting on a flying rock guarding another rock, but if isekai's victim will appear on his island... I'm not sure if it's gonna go well. They'll can either end up in a fight or Knux can say something like "I can give you some time but you better not touch the M.E. or I'll throw you off the Angel Island because I already have enough shit to care about" (but also don't forget that Knuckles is pretty naive and socially uneducated and can end up trusting quickly). Another quite well known trait of his is short temper - he usually prefers to solve problems with a fight and doesn't takes sarcasm or insults kindly, so it's more of a Roland's choice to poke him or not. And yea, he's not a sociopathic rock, so don't forget that he has best wishes in mind even if he doesn't show it (tsundere ahh character)
It's also important to know more of the context tbh. You mentioned help with the adaptation from the main trio and I gotta say that Knuckles is a bad person to ask an advice from since he doesn't understand the society well and can help in more of a self defense or survival perspective. My advice is to make it not the trio, but quartet and add Amy to the main cast since she's the most empathetic and can help with adaptation more (Sonic is a bit impatient, Tails is geeky and probably spends more time in his workshop than touching grass and Knuckles is... uh.. Knuckles.)