any Uchiha worth his salt- Sasuke, Madara, Obito, Shishui- will break tsukuyomi.
apart from that, i don't think Tsukuyomi is breakable. descendants of the sage might share the same aptitude as uchiha do, but i really doubt it and anyway there's no proof to support that.
however, while there's no cancelling the effects of tsukuyomi, I do think it's possible to tank it. the main thing to keep in mind is that it's a jutsu which targets the victim's will. meaning that the if hte victim's will is of a strong enough level theoretically it's possible for him to bear it until itachi's chakra gives out, maybe. i have some proof below:
For one, Itachi using one day on Sasuke and three on kakashi shows that there's a direct correlation between the level of tsukuyomi and the level of the opponent. ie. it takes more days of torture packed into a second to subdue a higher level opponent.
secondly- Sasuke slipped into a coma immediately after one day yet Kakashi was still conscious for a while after suffering three days
thirdly- itachi was really fatigued after three days of tsukuyomi on kakashi and not visibly fatigued after using 1 day on sasuke. meaning he cannot increase the number of days without severe risk to himself both stamina wise and also putting him closer to blindness. which means the number of days is probably capped under a week, three days might even be the highest level but since there's a bit of a doubt on why kakashi didn't die or not it might be a bit higher.
fourthly- there are characters who've made incredible durability feats like kisame and jiraiya. or others which have access to regeneration like kabuto, tsunade,naruto and hashirama. the will of the person plays a lot in this scenario.
in short, some characters may be sufficiently strong to wheather the effects of tsukuyomi and keep on fighting, and are strong enough that the damage they take isnt that compromising compared to the stamina loss/backlash itachi takes.[/QUOTE]
Eh? i don't recall Itachi being "fatigued" after the 3 days at all... after any use of Tsukyomi it puts a strain on his eyes and then it goes away after a while.. this strain is seen no matter what time he puts on it
Ps- It's impossible to tank something when he can up the time to what he wants, as he said, he has no limits in that realm, he's god and controls space, time etc
Also, there is no doubt as to why Kakashi didn't die.. Itachi let him live, which is why the 2nd databook stated he was Merciful, as that's the only moment he showed mercy to anyone in part 1
