Well, sometimes there is more than one right translation.

Technically, "That guy is I" is correct; in practise, no-one would ever say that except perhaps someone who learnt English as a foreign language and thought that "me" is always accusative case. In fact "me" acts as accusative sometimes and as emphatic nominative at others (all the accusative pronoun forms are also emphatic nominatives), but because it can never be the subject of a verb you will hear things like "Me, I'm ...." or "I myself am ..." (because the non-emphatic nominative forms are generally not emphatic when standing alone). Even in the second person plural, although the nominative subject and the nominative emphatic are the same, you need to use either the emphatic form before the subject ("You, you're ...) or use the reflexive pronoun after the nominative ("You yourself are ...").
So "That guy is me" is good English, but "That guy is I" is not.
"I'm that guy" (or "I am that guy") is also good English.
Why did I use both? Well, now that you've mentioned it I remember what happened: I began translating and got to "your look saying" and was wondering whether to include "that" although there's no "que" in the Spanish and think that I didn't want "that that" so I should turn it around and have "I'm that guy" when my wife dragged me off to do other things - things like stopping translation in midsentence happen now and again. When I came back to it later I remembered thinking of "I'm that guy" and used it from then on, having forgotten that I started out with the other translation of the phrase and that the reason for using this version applied only to this point, and even forgetting that I'd decided to put the "that" in despite the absence of a "que". Obviously I'll look at it again and mabr change I need to fix the odd typo too ("yoy" for "you", for example).