Mentally challenged people marry and have children- but in his case his IQ is genius level so he wouldn't legally considered mentally challenged. By the end of KMG he doesn't have a lot of emotional experience as an adult and he is innocent but he's also obviously sexually interested and in love. In terms of innocence he's probably not that much more innocent than some overly protected young bride.
The more concerning issue is that his life is so tragic. He gets a few (if that) years of happiness until Nisha dies and he's kidnapped and tortured for 25 or so years. He gets his son and mother back for a short while and then dies. If there must be a Krrish 4 then they better fix this Rohit mess. 🥱
HR's character in the movie itself is unrealistic. It's hard to judge his marriage. There is no character or sane personality development except for the magical increase in IQ,
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