Board Thread:Questions and Answers/@comment-81.167.2.56-20170304233306/@comment-27929130-20170305195234

The fact that he has tatoos may be the same reason why he's able to shapeshift, but that's just a crack theory. The kagune needle thing is pretty likely, but honestly, I think that this is another one of those mistakes that wasn't thought out, like when Kaneki stabbed himself with a kitchen knife and nothing happend but was later cut by a broken teacup.