1. Both Heartfixer and Dark Sonic are corrupted/evil versions of the protagonists of their respective franchises. Marinette for Heartfixer and Sonic for Dark Sonic. 2. Both transformations were caused by the character experiencing extreme heartbreak, for Marinette, it was the misunderstanding that she was cheating on Adrien with Nino that was faked by Ray and his crew when they were Akumatized as the Couplewreckers. As for Sonic, it was when he saw Chris and Cosmo were captured by Black Narcissus. 3. Both transformations were caused by a negative power, an ultrakuma for Heartfixer, and negative energy from fake Chaos Emeralds for Dark Sonic. 4. Both forms look very similar. With the character turning completely black or magenta with white glowing pupil-less eyes. 5. Both Heartfixer and Dark Sonic were shown to be extremely powerful, as Heartfixer seemingly could fix broken hearts, and Dark Sonic was faster and stronger than normal Sonic ever was. 6. Both transformations didn't last very long, lasting about 1 or 2 minutes. 7. Both characters snapped out of it thanks to someone ironic reasoning with them. For Heartfixer, it was Adrien despite the fact the misunderstanding between them was what caused her to get Akumatized in the first place. And for Dark Sonic, it was Eggman, which is strange considering he's Sonic's nemesis. If you know any other similarities between these two, let me know!
(c) Miraculous (c) Sonic The Hedgehog
