The Normal Arm is the only weapon you can use in and out of battle situations. You can’t upgrade it whatsoever, and you have it on you at all times. You can choose to go without weapons at all, and only use this, but I wouldn’t recommend it unless you really love a challenge (or are insane).
I’m not really sure why the word ‘arm’ is present in the name, because kicking is the sole function of this weapon. Megaman swings his foot forward and hits anything in range. The damage is pretty small on its own, but it’s more of a utility weapon than an attacking one. You can use this in quite a few places: kicking small bombs back at reaverbots (or Bonne machinery), flipping Guruguru over, kicking soda machines for a free beverage, checking trash cans for items, kicking cans into a bakery, or even attacking small animals (a certain small dog comes to mind).
Its uses are a tad bit limited, though. While it is nice to kick bombs back at the opponent, they really don’t hurt enough to warrant using it, although it is fun. Flipping Guruguru over is still pretty useful, although a few buster shots may be a safer route due to the rather low range of the kick. Also, the Shield Arm does those same two above functions, and it actually does it better. You’re also pretty unsafe trying to hit anything with this is general, as the low range and attack power really don’t make this worth using. I will note that kicking burned Servbots does produce energy cubes, which is very useful (despite how wrong that actually is). Try doing this if the Marlwolf is giving you trouble.
Leg Arm is more of a novelty weapon than a useful one. It doesn’t really have too much usefulness in battle, but it does have a lot of humor value in it (Paprika gets some distance!). I’ll have to end up giving the normal arm a 4/10.