This is the first boxtroll that Arthur meets. All he knows about them is that they're "timid creatures" and they "loved everything mechanical" (5.2). But he doesn't run into them much underground, because they're kind of shy. But when Arthur first sees Fish, he's smiling, so that's a good sign, right?
In fact, Fish takes Arthur by the hand and leads him in to meet the others. That's some pretty friendly behavior right there, including when Fish "turned to him, squeezed his hand, and made a happy gurgling sound" (5.14). Fish also takes the new boxtroll, Match, under his wing when the mini-critters show up at Willbury's place.
Fish seems to do well with new things in general. When they're making their getaway from the Cheese Hall, Arthur hangs on to Fish and they fly to safety together. At first, Fish seems freaked out—we're told "Fish let out a howl" (26.43)—then he gets the hang of it, "smiling" (26.46) at Arthur. A flying boxtroll, who'd have thought?
As another example of Fish + new = good, we see Fish being the one to figure out that the police don't pay any attention to underlings. He leads a procession of boxtrolls off the ship and returns with food for everyone, which is no small accomplishment. Well done, Fish, and three cheers.