Although Mai came to Mizuryukei Land due to her friends invitation, she is unable to muster the courage to join in the raunchy acts going on all around her. When Mai gets on the "Fuck-ris Wheel," where random couples meet, she reunites with Yuuta, a boy she met on the shuttle to the Park. The two are shy at first, but slowly tell each other their honest feelings and...

Who is the smartest anime/manga character?

I second Akiyama from Liar Game as being a remarkably intelligent manga character that manages to stand out in a cast full of borderline geniuses. I would add Orihara Izaya from Durarara!!.
When it comes to scheming and predicting behavior of virtually every single member of a very vast cast, few do it as well as Izaya.
Add to that a quirky but highly flexible way of thinking and a profound knowledge of human accomplishments and you have a mastermind in the making.
His plans do not always pan out but it is part of Izaya's genius that even a failure can be worked into a greater, overreaching plot. He is not the only highly intelligent in the cast but still manages to stand out. Johan from Monster is also brilliant and it is probably a blessing of sorts that his psychopathy, for all the grave harm it causes, never achieves a global scale.
If Johan were to truly put his mind to it he could probably easily bring about the end of the world as we know it. Ohtori Akio and Souji Mikage from Shoujo Kakumei Utena are also well worth mentioning.
Only Mikage is 'officially' a genius (along with Miki but his intelligence is not as much on display as Mikage's) but Akio is in many ways more intelligent.