The guide in Berkeley states:
The personal statement should give concrete evidence of your promise as a member of the academic community, giving the committee an image of you as a person.
So what tone should I use here? If as a person is as yourself, then is it better to use informal voice, like writing in a blog? Or should I keep the formal voice as most statements require you to be professional?
In my opinion (and three years working in the admission committee), it's less about the tone but the arguments and evidences you cite in the statement.
For tone, keep it overall professional and formal, do not use slang and do not write to them as if they're your high school friends. Write as if you're writing to a teacher you respect.
For your arguments and evidences, I'd suggest focusing on:
The decision of whether to include some personality traits of one self is really situation-dependent. But if you want to let the committee know that you're a "good communicator," remember to provide concrete examples as well.
Hope this helps and good luck!
... the answers there kinda encourage to use informal tone. What do you think?
To me, being "informal" can mean, among many things, use of slang, figures of speech, and conversation-like writing style. Some of these do add color to the personal statement but I'm reluctant to endorse the statement that one should go informally without clearly listing what kind of informal speeches are to be used. Even within figures of speech, some are more acceptable than others in a personal statement. "Deer in headlight" may be fine; "a kick in the balls" is probably not.
I'd suggest, as I have said, don't get bogged down by the tone. Be your polite and eager self, and make a case on the three points to persuade the committee members. Treat them like someone you respect and it should be fine.
I guess that the guide encourages applicants to not hide behind a neutral tone. You should "be yourself" and write in the way you normally talk to people in the workplace.
You may consider reading your personal statement (or asking someone to read it), and then try to assess the tone and imagine how you could describe a person who wrote it. Can you say something about the author which is not already explicitly mentioned in the text? If you can not see a specific tone and can not guess anything beyond what is already written, then the personal statement is probably too neutral.
For a personal statement, I would recommend speaking in the first-person and using a tone that is sufficiently informal to make it sound like you are talking to someone, rather than talking in the third-person about the skills of an applicant. Don't go overboard with the informality ---don't swear or use slang--- but make sure that the reader feels they are hearing your normal voice (which just so happens to be brilliant and articulate). The substance of your statement will be more important than the style, though the latter is part of showing that you can communicate clearly and in language that is suitable for presentations and academic work.
