Antiwhippy
the holder of the trombone
In terms of the "joke" trait, yeah it's hugely disappointing. As I recall, every character has one joke trait that is hammered into the ground by the end of the story. Y'know, meat, snrk, Teddyisms, etc. But I do think Kanji is more than that, just like the other P4 S-links. (Though I'll be more disappointed if Kanji's S-link goes nowhere.)
But the thing is that he's reduced to nothing more than a gay punchline in the main story when those interactions happen, and it does a lot more harm to his character than teddyisms, Chie's meat or yukiko's laughter. I actually find it pretty offensive with the way the main story treats him.
Like, being told that someone is afraid to sleep in the same area as you because they are afraid that you might rape them fucking sucks, and it shouldn't be responded with "I'LL SHOW YOU I'M NOT GAY."