These four masked monsters each corresponded to one of Kakuzu's four hearts.
Five hearts, five chakra natures: wind, fire, water, lightning, while Kakuzu's own heart was earth.
All five hearts could gather inside his body. In that state, whenever Kakuzu suffered a fatal injury, one heart would burst. In other words, Kakuzu had to be killed five times in succession to die for good.
The hearts could also separate from his body, like these four masked monsters.
These monsters had amply demonstrated their strength in the past minute.
In less than a minute, a squad of six Konoha Ninja had already gone to meet the First.
And one of those six had been an Elite Jonin! He was the first one killed instantly!
That alone showed Kakuzu's strength!
Fortunately, those six ninja had held Kakuzu off for a minute at the cost of their lives, allowing reinforcements to arrive in time and preventing him from strolling through the Konoha Gate unopposed.
Although Konoha had strength in numbers, with dense crowds of ninja converging on the entrance from across the village, Kakuzu did not panic at all.
The main reason was that those masked monsters had no fear of injury. As long as the hearts at their cores remained intact, they could continue to exist. Even if they were destroyed, it was only a matter of stealing another strong fighter's heart, something Kakuzu could do with ease.
So rather than avoiding the enemy's initial onslaught, the four masked monsters charged forward themselves.
It was precisely in a battle like this that the advantage of instantly casting Ninjutsu without hand seals was displayed to the fullest.
Although Konoha's Jonin needed only a few seconds to form the hand seals for a Ninjutsu, even a single second made a qualitative difference compared to