"The forgotten swordsman" is by far one of the greatest episodes of the 2012 series, if only because of it revealing so much of Karai's past in Japan.
Toshiro is Karai's sensei from the IDW comic book series of TMNT, he is the polar opposite of Tatsu and I could see the two of them clashing over Karai's training.
Karai and Toshiro