Sakurai said the reason it’s Min Min and not other characters is pretty simple: the Arms producer Mr. Yabuki said he wanted Min Min.
Min Min is strong against heavy fighters but has trouble with lighter characters and those who excel in mid-air combat. Min Min’s basic directional attack is just her ARMS extending, and the same for her side smash. You can actually bend the extendable ARMS, jump during the attack, and even execute a smash in mid-air.
If you tap “A” quickly instead of holding the button, Min Min performs kung-fu style kicks. These act as Min Min’s neutral attacks, and they can be combined with movement for a dash attack, an upward kick, and a kick that reflects enemy projectiles.
Min Min’s special is different from most. Pressing “B” extends her right ARM, where pressing “A” uses the left ARM. You can launch two attacks at once and create a number of combos or smashes. If you do launch two smash attacks at once, the left always goes out first.
The “up” and “down” “B” specials are different. The “down” special changes the right ARM out, with new extensions like the Ramram (low power, high range), the Megawatt (super strong, but slow), and Dragon, which is just a replica of the left ARM Min Min uses.
The “up” special lets Min Min jump using the ARMS, though Sakurai says it’s best used to escape from opponents instead of working as a normal recovery move.
Finally is Min Min’s Final Smash. Like the Dragon Quest Hero fighter, Min Min’s Final Smash summons all the other ARMS fighters for a full-on assault.
Min Min isn’t suited for close-range combat because her attacks are easy to dodge, though her direct attacks aren’t affected by counters. Since the ARMS don’t extend that far, you’ll want to be careful from fighters that attack from above.