Yi Sang is sad. That is what he comes off as. Like one of those ducks who has oil slathered between their wings, pathetically futile cries escaping the maw of a being not strong enough to lift itself any longer. No, I have nothing against Yi Sang. He's simply a funny guy.

 Yi Sang's voice is of medium pitch—he speaks softly, with slight rasp to his tone. I have always envisioned him a 'depressed poet' type of voice; he doesn't speak with emotion, rather sounding logical and observant... yet weirdly melancholic. 

Yi Sang voicelines/reference can be found here.