The question is well put. It is indeed a special case of how 連体修飾 (the relative clause) is used in narrative.
Linguistically this is quite simple to derive:
ハッキーは４人を救出した 'Bucky saved four people' transformed into
４人を救出したハッキー 'Bucky, who saved four people'.
What is interesting is the use to which this particular structure is put.
４人を救出したハッキーであった... is literally 'Was Bucky, who saved four people...'.
But this doesn't make much sense. I think Hyperworm's rendition of this as 'So there was Bucky, who saved four people' is a good one. It could also be rendered 'So here we had Bucky, who saved four people'.
As a narrative style, this is not telling us that Bucky saved four people (which we presumably know already). It is telling us something about the situation -- here was Bucky, this guy who had saved four people. As Hyperworm says, this little sentence is a prelude to further information, namely, that this was actually a trap.
This is a device commonly used in certain types of Japanese narrative. To make up a silly example (sorry, it's the first thing that popped into my head) using the present tense rather than the past tense:
Here we have Jiro who hates ugly women, and he's about to meet the ugliest woman in Tokyo.
This is setting the scene rather dramatically for what is about to happen. Of course we could say in Japanese as well as in English:
Jiro hates ugly women, but he's about to meet the ugliest woman in Tokyo.
But this would lack the dramatic flourish that the use of 連体修飾 (relative clause) provides.
Hope this clarifies what this structure is all about.