I think Big Boss was well suited to being a mysterious legend (and villain) of the series. In fact, I'd respect his character a lot more if I knew less about him, because in every single post MGS 3 game, he was always shown to be depressed and lamenting over his dead mentor. I mean, I get it; he's sad that The Boss died. But, there's no need to drag out the same narrative for the whole damn series.
Kojima has seriously mishandled both Solid Snake and Big Boss. Snake lost his personality in MGS 4 (as you said Ryan), while Big Boss is completely lacking character development; he still cries for The Boss up to the end of MGS 4
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