SkunkyArt | ZachaRicO — So Much In Common

They both wear outfits that are primarily blue but accented by something red. (Interestingly, the red accents are opposite each other: Sonic has his shoes, while Ash has his hat)

They both have a wild hairstyle. (Sonic has his spikes, and Ash has his hair)

They both wear gloves. (A bit of a stretch, but it's still valid)

They both have cocky personalities. (Though there's a notable difference in portrayals on whether this is good or bad. In Sonic's case, his cocky personality is what gives him his charm most of the time. In a sense, Ash's cockiness is also what gives him his charm, but it's played more as a detriment early on in his series. These days, he's not quite as overconfident, though he's still got a rash edge to him. But really now, would we have it any other way?)

They're both stars of cartoons.

They both have obscure puzzle games based on their cartoons (Thanks, PTM!)

They both have best friends with primarily yellow coloring. (Also a bit of a stretch, but still valid)

They're both primarily based in video games. (Sonic is a straight-up video game character, while Ash is a counterpart to the hero of Pokemon Red and Blue)

They both debuted in the 90's.

They both have a few best friends that their fanbases generally associate with them. (Sonic has Tails, Knuckles and Amy, and Ash has Misty and Brock)

They both have archenemies who have a reputation of serving more as comic relief than an actual threat.

They both have died at least once in their existence but don't remember the circumstances under which they were killed. (Thanks to PuffyTopianMan for this one: Ash got turned to stone in Mewtwo Strikes Back, and Sonic got impaled in Sonic 06)

They both have at least one female friend who has a canon crush on them, but it's unclear whether or not they return those feelings. (Sonic has Amy, NOT ELISE, and Ash has Serena)

They both have gotten design changes over the years. (Not nearly as noticeable with Sonic as with Ash, though: Sonic got little tweaks, while Ash's outfit has gotten an overhaul with each new series)

They both have gotten hate from their fanbases for one reason or another. (Sonic got hate for some of his terrible games, while Ash gets hate because he can't win in the league)

They both hit a low point in their respective series and rebounded from it. (This one's kinda ironic: Sonic hit his low in 2006, at which point the Diamond and Pearl Series, arguably one of Pokemon's best series for Ash, started in Japan. Then in 2010, Sonic rebounded with Colors...and Ash hit his low in Black and White. Come 2013, Sonic hit another low-ish point with Lost World, and Ash rebounded HARDCORE in XY)

They both got design changes that nobody asked for. (Sonic got his Boom design, while Ash got...plastic surgery. Funnily enough, both of them were met with mixed to negative opinions from fans at first, but their bases eventually warmed up to them for the most part, though animosity remains)

Both of them returned to their classic design in 2017.