WalkiesG 5

Flash can be purchased for 7,000 coins in the final tier of gadgets. Follow whatever route you like to get there, whether that's purchasing the cheapest gadgets to unlock the final tier the fastest, or purchasing the gadgets you'll find most helpful to generate coins faster. Once you have Flash, accumulate 10km of distance with him - despite the achievement description making it sound like this requires a single 10km run, this is a cumulative achievement, not a 1-run achievement.