pretty nice
I like it Idea is great sprite animations are great but i would to see in full version few enhancements like :
1: this is turn based game so maybe you add some rpg style gaining experience lvl-up attacks etc. etc.
2: add some tactical moves (for now u have done 2 attacks type strong & stronger) to characters for example some attack can stun enemy some other heal ur npc`s or enable . Sonic is very fast character so he can have few very powerful or stunnable moves
3:add new bar which filling when taking damage liek in some fighting games when full it enable character to do uber attack for sonic & shadow uber attack can b temporary change form for few turns into golden sonic & golden shadow new form can enable them to use a new move pool (you can move here overdrive attack)
4.add defend skill what enable to reduce received damage and gives percentage chance to avoid enemy`s bonus attack effect (stun etc.)