And boss #3 is defeated. Gray Fox was easy and I never died during the fight at all, but I still had fun. Although I couldn't help but think "Final Fantasy" when he said something along the lines of "I have come from another world to duel you".
Afterwards, Naomi Hunter revealed the disturbing story of what happened to Gray Fox after his supposed death in some earlier game in the series (Metal Gear 2? Just to ask, a good number of characters from this game made their first appearance there, no?). Then Otacon gave me just what I wanted: more info on Metal Gear REX.
Despite the long cutscenes, this game pretty much has a 10/10 on story locked - I'm hard pressed to think of a flaw otherwise. Unfortunately, my parents were trying to talk to me while I was watching that cutscene, so I may have missed some important info. From what I remember, the gist of it is that Metal Gear REX was created by Otacon to serve as defense against nuclear missiles, but then was turned into the exact opposite by President Baker. Yikes! Still very intriguing though. And then Otacon revealed his family's involvement in nuclear weapons manufacture. Everyone in this game seems to have some disturbing (but very well told all the same) backstory.
Now that I have a level 4 keycard, it seems that I'll have to backtrack to find new areas. I always enjoy Metroidvania elements like this, though, and MGS has been no different.
MGS2 is more linear from what I've heard though, although I'm looking forward to its gameplay improvements quite a lot.