@Rockdrop @Ore This is from memory, so it could be not accurate. There is a hilt on the Japanese sword, and exists to prevent the holder's hand from slipping forward.
The reason there is no guard is due to the mindset of the time: they're supposed to be efficient at killing. A bigger guard would prevent an efficient and quick unsheathe of the sword, which would put the opponent at an advantage. Those weapons are purely offensive, there is no thought about defense (a Klingon would be proud). Beside, parrying with that piece of bending steel wouldn't be a great idea.
The Gladius too didn't have a guard, because it wasn't needed. The weapon was somewhat secondary: they would have a pilum or another kind of spear as main weapon, defense is the scutum (big shield), and they would be equipped with great body armor.
Anyhow whenever you see a weapon and ask yourself a question, the first thing that should come to mind is: how was it used, which purpose?