I teach ESL and the way I explain it to my students is that "guy" (singular) is almost exclusively male. Where it's potentially confusing is the plural. "Guys" can mean either "group of males" or "any group of friends or familiar people" depending on how it's used.
- Do you see that guy over there? (Male.)
- The trouble with guys is that they never listen. (Male.)
- Hey guys, look at this! (Friends or familiar people.)
I'd be interested to know if anybody can come up with a real example (used by someone else) of "guy" (singular) not being male, or even where the non-male plural meaning is not used as a form of address.
- out with the guys (sounds "male" to me.)
- the guys aren't all here yet (also sounds "male".)
This adds a wrinkle to the claim that "guy" is "ulo" - but yes, it has a similar level of informality, I would say. For sure, "guy" does not work for "knabo."