Spectre-3, reporting for duty!
Chopper is the resident droid of the Ghost, assisting the crew
in everything from ship maintenance to combat, even though he doesn't always want to. After many years of repairs and patch jobs, Chopper has a beat-up, worn look that sometimes matches his cranky personality.
"Many years" is totally under-selling it! He's several decades past the usual expiration date for his line, and was active during the Clone Wars - he was a navigational astromech in a Y-Wing that was shot down over Ryloth (homeworld of the Twi'leks), and young Hera pulled him out of the wreckage and began rebuilding him. So he's cranky? His old master died in combat and he probably thinks it's his fault; of course he'd be cranky!
Chopper (or "C1-10P" as he's properly designated -
notice how the two 1's separated by a dash sort of looks like a capital H) is a short, squat droid, about halfway between R2-D2 and BB-8. Boy, pity the poor starship designer who has to create a single astromech socket that can accommodate all those different kinds of droids! The body here is a cylinder, while the head is a flattened cone, like a shorter version of an R4 unit. He's even got an R4's three eyes, rather than an R2's one.
Like a lot of things in Rebels, Chopper's design is based on Ralph McQuarrie's original Star Wars concept art - so like the concept R2, Chop's got little arms sticking out of his neck, rather
than just getting by with three legs. Those are articulated, allowing them to hinge out of the head or be tucked away when not in use. There's a third arm that folds out of the chest, and features an extra hinge in the middle. There are panels sculpted to be missing on his chest, and his legs are different styles. Instead of a foot that drops out of his body, like R2 has, Chopper has a wheel. Sometimes he'll just throw his legs back and speed around on that gleefully!
Considering how often Sabine repainted
Chopper in Imperial colors so he could infiltrate a base, it's weird that they kept reverting him to the way he looked before: mis-matched colors randomly scattered around, rusty spots, etc. Maybe he liked the way he looked, and insisted on being put back to normal? If that were the case, though, why did he once bail on his friends when he saw the opportunity to go shopping for a left leg that would match his right one?
The toy includes that alternate ambulatory strut: the left leg is removable, and can be replaced with the upgraded version. Additionally, you can take out his wheel, and swap that for the rocket booster he uses to fly; he won't be able to stand with that in place, thanks to the large jet of flame sticking out the bottom; however, he will be able to fly, since the set includes a big cloud of flame and exhaust the jet can plug into. It's a really cool piece, and does a lot more to justify the price here than the Black Series R2-D2 ever did.
If R2-D2 is the family dog, smart and loyal, then C1-10P is a cat: self-concerned and destructive. His lifetime of rough repairs have left him with a distinctive "grumbly" voice that further sets him apart from the friendly R2, though he did eventually get his own C-3PO expy in the form of AP-7 (a "Death Star Droid"
from the era before there was a Death Star). While this figure is based on the Rebels cartoon, Chopper made a physical appearance in Rogue One, and the toy design is untooned enough that it could easily pass for a live-action version, as well. This toy isn't just fun, it's versatile!