Originally Posted by T-Mar

Not knowing the brand and Not knowing about the brand, are two entirely diiferent things!

A drive side photo may help but I see a steel, cottered crankset, steel handlebars, brake safety levers, steel rims, nutted hubs and fork blades that are crimped to form dropouts. All these all characteristics of very low end bicycles, the type normally sold in department or hardware chain stores. As for age, the graphic and colours scheme suggest late 1980s.

I'm only aware of two Fisher brands, Gary Fisher and Fisher-Price. It is obviously neither. Based on the OP's location (Greece) and the apparent level and era, it's likely a European brand owned by a company who were having the bicycles contract manufactured and sold through some store chain, possibly their own.

A drive side photo and serial number may help to narrow down the age and possible identify the manufacturer. Hopefully, there are some European based members who may have familiarity with the brand.