Board Thread:Questions and Answers/@comment-66.87.79.104-20140714071241/@comment-58.166.97.27-20140724174452

Actually, that's a good question - Seijuurou and Nitori don't have confirmed animals as of yet, (even Sosuke is confirmed to be a whale shark) however I've seen people use the narwhal and the marlin to represent Seijuurou (while Nitori gets the remora because the remora fish stick to sharks a lot, and we all know how much Nitori likes sticking around Rin).

The narwhal because it's a type of whale (and Seijuurou, Makoto and Sosuke are confirmed to be the tallest and largest out of the cast, though I'm not sure who's larger between Sosuke and Seijuurou, though I'm willing to bet it's the latter), and they have a tusk that, though the point of it is not really confirmed, is speculated for several things such as using it as a weapon, to maintain dominance amongst the male social hierachy, and to finally attract a mate.

The marlin, is sometimes used because it's a sporting fish that swims at a very high speed (sometimes considered the fastest in the ocean - alluding to Seijuurou breaking a tournament record, and he was able to close the gap in his final relay against the Iwatobi team when he swam butterfly against Rei), as well as an apex predator (again, alluding back to him going neck-to-neck against Rei, and being able to close the gap). They're also highly sought after by fishermen, and basically, if you catch one, you're considered badass (and let's face it - Seijuurou is quite the badass).

Personally, I like using the narwhal as his animal representative. :)