ترجمة - إيطاليّ -انجليزي - Frank Searle era più solitario degli altri...

"[...] era (il) più solitario" would be "was the loneliest".

Frank Searle was lonelier than all the other monster-hunters.

The mistake is in the grammar: it's a comparative not a Superlative.

I agree with Cammello, "lonelier than" is correct

I agree with Cammenllo too!