When he talks to himself, Yell "Stop that, it's weird."
Every time he says "Eliminate", poke him.
While following him, greet random people as "Kami" Until he gets really annoyed
When he's writing in the Death Note, look over he shoulder and gasp every time he writes
Skip around him, a lot
Throw chunks of ice at him
Steal his glasses and don't give it back until he begs for them
When he goes on Kira's Kingdom, run on stage and grab the microphone away from him, then start complaining that you should have been given the task of judging people, not "that dude over there" and point at Mikami.
When he goes to the gym, ask if he thinks he's fat
Make him take you to the ice cream store afterwards
When he gets mad at you, cover you eyes and run around screaming franticly, "You can see my face! You can't kill me!"
Hugs him at random moments and say, "Poor Emo kid. You need a hug."
When Gevanni is following him, walk over to Gevanni and tell him Mikami has a gun and is holding you hostage
Tell him he's a pretty boy like Mello
When he asks who's Mello, slap him and tell him to shut up and that he should know
Change his calendar to do random stuff, out of schedule
Skip down the street, holding his hand and singing
When not skipping, push him at really inconvenient moments, like waiting for a train
When he can't kill Near and the others at the warehouse, tell him he fails
When he sees Raito, tell him it's a lie, that you are God.
Jenisa: OMG, Mikami IS a pretty boy.
Me: I know, that's why I put it there
Jenisa: Hey, you got inspiration from that comic.
Me: shhhhh!! Don't tell people that. I don't want to be sued!
Jenisa: So what's with the ice theme? Ice cream, ice?
Me: It was kinda hot when I was writing this. I was craving something cold.
Jenisa: So, you wanna go hug the emo kid?
Me: Yes! Let's!. runs off to go bug Mikami
There's not as much. Mikami isn't a MAIN character. We don't know that much about him.