When referring to an online comment, I want to say
I agree with this guy.
However, it's not gender neutral (See this ELU post.)
What other informal alternate word is there? 'User' seems too formal, so does 'person' or 'fellow.'
You could say
I agree with the commenter
I agree with them
I agree with this person
In fact, possibly the best is use their name (Kudos @Mynamite)
I agree with @jack
You can also use I agree with 'em. It uses singular they which is gender-neutral, and the omission of th gives it the air of informality requested. Double bonus that written it can't be confused for masculine-only 'im like it can in speech.
If you really want to use this in your sentence, you could go for an informal form of I agree with this one, although to be honest I can't think the best way to write it. with this'n will get the right pronunciation, although perhaps with this 'n' would be the most formally correct (it's a syllabic n after this in speech) although likely confused with the 'n' of and.
One possible option would be -
I agree with him/her.
Or even the parenthetical (s)
In my opinion, (s)he is correct.
But I think you could possibly omit the person in your case, using the terse
Or, the simple
Or, the more complex double negative
I do not disagree.