Reading on a topic about Hsu Hao, the infamous Mortal Kombat loser, I saw one comment about characters like Rain and Noob, how Netherealm was able to improve them. Someone else pointed out that the two were already liked as cool recolor ninjas. This reminds me of something: Perci.
Perci was liked because of her appearance and description. She looked a lot like Julie-Su, like how Sticks looks like Marine. Hence, she was shipped with Knuckles. She didn't do much in Boom proper, but she has the second-biggest following of any Boom character. Sound familiar?
A good extra can spawn a decent following.