Permissions

Alright, so. Sei here is kind of a ridiculous jackass, but he is also capable of sensing other magical beings and, to an extent, identifying them. For instance, in his canon, he could tell Castiel Some Angel Being Who Isn't Actually In This Game is magical, and an angel, but he couldn't tell his name or angelic rank. He cannot sense things that aren't canonly stated to be magical in nature: For example, he can't sense Gene X, no matter how ridiculous the mutant's power is.

Secondly, if you catch Sei singing, he basically has the effect of a giant overgrown tribble, with the same sort of soothing effects and less the ridiculous breeding.

May Sei sense magic on your character? How much do you want him able to tell?
May he use this information to be a jerk to your character?
May Sei's singing voice have a soothing effect on your character? How much?

All of these are entirely opt-in, so if you don't want him to or aren't sure right now, he won't!