Most sonadow fankids are purplish and tube-grown, so I decided to make her neither!
Background for the story under cut, in case it gets long!
So in this AU, stories from the more recent games happen when Sonic and Shadow are at most around 20 years old. Back then Sonic was rambunctious and reckless as we know him to be, and Shadow was a depressed, traumatized emo kid. While at first they were rivals as depicted in the games.
Over the years tho, just like their friends, the two matured. They started getting along a lot more, and became official friends, which eventually lead to dating. They both were afraid of getting serious though, so they took things slow and casual. Due to some of their unresolved issues, they couldn't get any closer. But not for much longer.
Some of it has been brought on by a Particular Event, and some of it due to restructuring how they go about saving the world, but things changed. As Forces has shown, putting so much responsibility on One Guy isn't wise. Sonic felt so quilty for failing, but it never should've been his job alone, so they created an organization, to share the load more evenly!
The org became global, with local groups of heroes being recruited all around, meaning that now the characters we know and love now had more time to take care of their personal lives.
While for some the change hasn't been much, Sonic, and to a lesser extent, Shadow, struggled quite a lot. Many of things happened in between, but eventually both of them grew to quite enjoy the domestic life.
They moved in together, and started taking things to the next level, now that both have done some healing and therapy. They found that they get along quite great. After a bit longer, they decided to start a family :)
Shadow came up with the name. He wanted to honor his sister's memory, but Mar-Mar was also a symbol of all his progress. He did go to therapy to make sure that he can handle them sharing a name, to see if it's a good idea, and well, it was!
She's now 4 and very loved, and her dads love each other very much as well. For a certain Sonic though, things won't go so great.