@bibibbon I was reading your post and thinking when you mentioned Mummy, and certainly is sad and disappointing that out of Nine's crew he was the one to get the short stick in terms of screentime and interactions with his own leader.
Such a cool looking villain with an awesome power shouldn't be treated like fooder.
But eh... I guess they really need give Bakugo an easy victory to show how badass he can be compared to the other loosers from the class.
Anyway, before getting absorbed by my hater side I can share my ideas about rewrite Mummy, because like his friends is fun write about him and fill the empty space left by the author.
Starting for his backstory:
This idea isn't entirely mine, but I love the concept of Mummy and Edgeshot being related.
Interesting to think because they share same birthdays, both their quirks have the aesthetic of red tape, they have grey hair and the names "Kamihara" and "Makihara" sound strangely similar.
I've seen people write them as twins, but I like more the idea of them having a big age gap in case of being brothers which fits more with Shinya being in his 30s in canon.
And so both Hoyo and Shinya being brothers born in a very traditional japanese family would make room for some family drama, as is more than probably that Shinya being the first born and with a quirk like foldabody would probably be seen as the pride of the family.
While Hoyo is seen as lesser and never able to live up to the level of his older brother, especially after Shinya got accepted at UA and started pay less attention to Hoyo.
While during his early childhood Hoyo only support was Shinya when he left the family state to focus on his studies Hoyo was alone with a family that never supported him and constantly remembered him how inferior he was.
Hoyo will reach a break point when his parents tried to get rid of him when he was entering on his teenage years, and he discovered they sold him to a mysterious organization that was looking for young low tier soldiers (which for some reason still happens in MHA world and nobody seem to care about it).
Note: This organizagion will be no other than the MLA but isn't really important for the story for now.
And so, without anything to lose since Shinya was long gone at that point, Hoyo escaped the family state and never came back. He also invented the false surname Makihara, as he didn't want anything to do with his family.
That would be the beginning of some rough years living in the streets for Hoyo.
Without proper studies (since he only got the basics at home instead of going to an actual school) and technically being a fugitive of his own family, Hoyo only could resort of stealing and other minor crimes for try to survive being homeless.
I imagine he also tried to gain some money doing dirty jobs for other criminals, which might be why we see him beating thugs in the street during the flashbacks.
That being said, this empty life style wasn't something Hoyo was happy about.
He still feel the necessity of prove himself and try to became something better, he was angry at the world and his family, and he also missed Shinya despite he didn't want to admit and prove his brother his worth.
Despite Hoyo's quirk still seemed to be quite useless as he couldn't do much with his puppets and give up at some point.
That was until he meet Nine...
My idea of how they meet is that Hoyo was hired to expulse Nine from the territory of a criminal gang that didn't like a homeless man like him was wandering in there.
Expecting just another quick job for a bit of cash Hoyo used his quirk with and old scarf to create a puppet out a vending machine, with he would use to intimidate the white haired man described by the gang.
Worse case scenario would be him having to beat the man in case he resists but isn't like he cared.
What Hoyo didn't expect is that when he meet the white haired man this one would have an aura of calm around him and rather than look intimidated by him or his puppet, he only got a glaze of interest coming from him.
"That... looks like a very powerful ability you got there" Nine commented very calmly looking at Hoyo like if he already knew everything from him.
Hoyo was a bit confused by the statement (and internally a bit intimidated by the way the white haired man seem look through him) but he only wanted to finish the job quickly as he was hungry and needed the money the gang offered.
So he tried to attack the white haired man with his puppet, but in the blink on an eye a small lightning bolt hit the puppet and make it explode.
Hoyo was pushed back by the explosion and the white haired man was still there with an unfazed expression.
"Not that powerful yet but..." Nine said looking at Hoyo "I can make it stronger" He said like it was a promise.
Nine then walk away and Hoyo tried to follow him without understand anything of what was happen, until they were almost in front of the gang hideout.
Hoyo observed how the white haired man rising one of his hands at the sky, and he didn't realize how suddlenly the dark clouds covered what until some moments was a very clear night.
A lightning bolt came from the sky and impacted the buildings Hoyo knew the gang used to hide, making the ground vibrate as they explode illuminating the night with the fire and the white haired man was stading there with the same bored expression as he that act of destruction was nothing for him.
Hoyo was totally speechless by that demonstration of power.
"I assume they won't represent a problem for you from now, I understand they wanted you to get me out of here so you don't have to worry for not get the job done" the white haired man said with the same calm Hoyo was used at that point.
For some reason it reminded him to someone else...
"Huh... and what do you want from me?" Hoyo replied rising his eyebrown tried to show more confidence that he actually felt "This kind of "favors" can't be for free"
"From you? Nothing..." Nine said plain and simple "Maybe offer you something, but first we should get out of here, he police and heroes can be annoying and they probably were alerted by my little spectacle" He said before walking out of the scene.
Hoyo didn't know why but he started walking behind the white haired man. There wasn't much to lose anyway...
"Are you hungry? Personally I am, so maybe we can get some food" Nine said after notice the young boy was following him.
Hoyo agreed on that, he really could get something to eat.
...
After all that happened Nine started to explain to Hoyo what his ideals and dream to change the world, a dream he really wanted to share with Hoyo for some reason.
Even if he didn't believe he meet the standards of power the white haired man claim to have, but Nine keep insisting on the potential of his quirk and how Hoyo was stronger than he realize and how wrong society was for overlook his true potential.
Not much time later Hoyo meet a friend of Nine, a really intimidating mutant who called himself Chimera and that name was fitting considering he was a mix of countless animals.
Hoyo found the guy to be quite charismatic and funny regardless of his loon however, and he was also very compromised to follow Nine's vision for the ideal world he evisioned.
And one day Nine also surprised Hoyo with a gift: a lot of red bandages that he notice were more resistant and powerful than anything he previously used to activate his quirk.
"Your power is greater than you think Hoyo, don't believe what the weakelings tried to make you think all your life. You can be better than them, you can fight for the world we deserve" Nine said in the most genuine way Hoyo ever heaf from someone.
And Hoyo really felt weak but also strong for equal reasons that moment, as never during his whole life he felt supported the way Nine made him to feel. Not his own blood family, not even Shinya, but this man who only wanted to give him and Chimera a better world to live.
Hoyo knew that moment that his life and heart belong to Nine and his cause forever, he give him a real family and home without even expect anything from Hoyo other than fight together like a team.
It motivated Hoyo to push himself and his quirk over the boundaries, training endlessly with the help of Nine and Chimera, so what once was the ability of control a single puppet quickly became the power of create armies of puppet soldiers thanks to the gift Nine give to him.
He even discovered an inhate talent with the sword, a gift courtesy of Chimera by the way.
Hoyo was poweful indeed, just like his friends believed.
But he also wanted a new identity as well, as his born name no longer felt something that represented who he was.
Then Hoyo became Mummy, and he used the bandages gifted by his leader to cover his body.
He felt safe and powerful like that.
Mummy finally meet himself that moment and knew who he really was.
...
P.S:
-Despite I make Shinya look like a careless and selfish brother in the backstory, I actually think later when he found what his parents did to Hoyo he resents them and felt totally guilty for not being there for his little brother. Edgeshot look for Hoyo during years hoping to find him someday and be a family again, he also keep his parents out of his life after what they did to Hoyo.
-I don't mention Slice here because I think she was the last one to join the team some time later than Mummy.
-While their age difference isn't actually that big, I like the idea of Mummy considering Nine a big brother figure and somehow filling the empty space left by Shinya.