Is comma necessary?

Not in the sorts of situations in whih "Hi" is appropriate.

Could be elaborate on what you've said?

"Hi" is a mainly American informal spoken salutation like "bye" or "see you". If it is encountered in writing it is either in an extremely informal communication such as an email, where punctuation hardly matters, or in reported speech, in which case the rule is that a comma follows a salutation.

Hello, Mike.
Goodbye, Yoong Liat.