We don't really know who Buster is just from this title. He could be an ordinary person whose story has been told to illustrate a particular point about having a'mal heart', whether it's about his character flaws or some sort of misfortune he has faced.
Sure, She-Hulk is related to Hulk. Jennifer Walters, aka She-Hulk, got her powers because of a blood transfusion from her cousin Bruce Banner, who is Hulk. This connection is a key part of their storylines in the comics.
Captain America would be a great addition. His leadership and moral compass could guide Hulk and She - Hulk when they face difficult decisions. And his combat skills could come in handy during battles.
For Buster, he might start off being a bit intimidated by Montana Max's wealth and bravado. But as the story progresses, he could grow more confident in himself and stand up to Max when necessary. This would show his development from a somewhat shy character to a more assertive one.
Comics usually depict Hulk as stronger than She-Hulk. Hulk's strength levels are often at a much higher magnitude, but it doesn't mean She-Hulk is weak by any means. She has her moments and skills that contribute to her being a significant character in the comic universe.