Keisuke wouldn't like being in the public eye, and I'm actually uncertain if his politics align with mine. He hasn't responded when I ask and I'm not sure if it's due to indifference or uncertainty or something else. We agree on most issues socially and domestically, but I don't know about his economic or international views and I don't know if he even finds my preferred form of government acceptable. I'm also not sure when you say that "anime is real", what about anime is real now? Every single anime plot? Or just the characters? What about supernatural abilities and such? What if anime becoming real means that the Third Impact happened or something? (I'm just mentioning Eva because, you know, memes and it's accessible; there are plenty of other catastrophic possibilities, too.) There's a lot to think about and I'm not sure if I can take into account all the changes that might happen should anime become real.
I imagine if somehow we did become the rulers of our country, the world doesn't become completely batshit insane, and for some reason we aren't violently overthrown immediately, Keisuke would try to keep my fellow politicians/secret service/etc in check and as corruption-free as he could, as quietly as he could. Injustice matters to him but he wouldn't want to make a spectacle of it. I imagine he'd spend a lot of his time doing that, even though he's not particularly graceful with words (although politicians have definitely been getting blunter lately).
Outside of that, he'd likely focus on helping those in poverty, especially children who don't get food outside of school. He would also, after gathering some courage, try to handle our nation's drug problem in a sensible way, both encouraging those who completely condemn drugs to explore the reasons why someone might take them and also providing rehabilitation for those who need it.
All that said, this is pretty hard to imagine even without the rules I mentioned above because Keisuke doesn't seek out conflict, nor does he particularly like to be in the spotlight.