Characters are similar, with similar dynamics:
Natsu is an optimistic, adventurous, happy-go-lucky guy, like Sonic.
He has a companion without whom he doesn't go anywhere, Happy, which is similar to how Sonic behaves with Tails.
Lucy drops into the story very suddenly, but later becomes an integral part of the team, wants to stay with its members for as long as possible, and takes a liking to the adventurous main character, as does Chris Thorndyke.
Grey's a hot-headed, kinda aggressive person, who has constant conflicts with the main character, like Knuckles.
All characters have special abilities.
They have a similar tone as well (mixing drama with comedy regularly, with several action scenes).
The main difference is the setting.