スリッピーはちょっと気が弱いが、これが憎めないんだなぁ。 Slippy is a bit faint of heart, ...
憎む 【にくむ】 to hate; to detest
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Pretty sure これが憎めないんだな just refers to Slippy himself. Something like "You can't bring yourself to dislike him" either because he's cute or some other reason that makes him likeable. So "he is a bit of a coward, but you just can't bring yourself to dislike him."
The meaning of the phrase is, surprisingly, pretty literal. Here's a website that even translates it as "lovable".