So what happened to Akihito? Is he brainwashed or he forgot about Asami?

From the way Akihito woke up in the hospital, I’d say he forgot about Asami. Asami’s enemy took advantage of his loss of memory and convinced Akihito that Asami is a bad guy for whatever reason. This is just a guess since I’m sure multiple people have different theories.

If you're interested, there's a whole fanfiction on what might have happened :D
https://archiveofourown.org/series/2098347