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Humans who can keep up with a captain level shinigami, Ginjo might make some sense, since he IS an ex-shinigami.
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Yeah... for the most part, they're not even close.
Tsukishima is only able to keep up because of his ability. Since he can physically affect the past with it (and yes, it's the past rather than the target's memories, I already debated this with ILP), he also gains the experience from that. He could have been training as long as he wanted.
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Next is that he just keeps changing character personality according to his wishes, plus the past he makes for them is just so generic it isn't funny, he devices awesome abilities and then makes the stupidest techniques with it.
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This I will agree with, especially the first. Yeah, a lot of things he creates are stupid, but sometimes you just have to look a little harder. And yes, a lot of times he changes personalities (Renji is actually the biggest example), but most of the time it's not so drastic so as to be beyond reason.
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Kubo is afraid of letting main characters die, each time they just grow stronger and stronger and stronger and so does a new random enemy that appeared.
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This is my biggest issue with Bleach. Somebody needs to die. Like, badly.