Lars is a character briefly mentioned in Heather Report, Part 1.
Dragons: Riders of Berk
Ruffnut asks whether Snotlout found 'Cousin Lars'. This shows that he is a Thorston. Tuffnut, being the sadist that he is, asked whether he found 'Cousin Lars's' severed head. Fishlegs reacts to that, becoming terrified for Lars's fate, so it is likely Fishlegs is friends with Lars.