First I know that bone based quirks are common and aren't new/unique, I promise I didn't steal/copy this quirk or character. I made this OC for a table top game years ago and decided to make him a MHA OC. His original name was Marrow but there is already a hero called that in this sub-reddit. But despite that I hope you enjoy the character.

Name: Maxwell Cain Willow

Hero Name: Skull Sentinel "The Skeletal Hero"

Sex: Male

Sexuality: Straight

Age: 20

Appearance:

https://preview.redd.it/chm1zhawkyo51.jpg?width=1000&format=pjpg&auto=webp&v=enabled&s=39e0ba034a2bb51e36184f0ca53b75d55b533dc2

Quirk: Osteokenesis - Max has the ability to manipulate his bone matter on a molecular level. Allowing him to construct and shape the bone matter in his body to an unparalleled degree. His quirk also allows him to create more bone matter using resource reserves that his body stores for just his quirk. He can increase and decrease the density of his bone constructs, alter their molecular structure to make their sturdier, fragmentable, flexible, etc. He can increase the density and weight of the bones in his arms a split second before he lands a punch to make it even stronger. All in all his quirk relies on his imagination, ingenuity, and skill.

Quirk Downsides: His quirk requires lots of concentration for more complex structures since he manipulates the bone matter on the molecular level. Simpler structures are more second nature and thus require little effort to make. He also requires calcium supplements to fuel the bulk of his quirk while the other minor elements of bone matter are taken care of by an increased consumption similar to the Flash's Metabolism. Creating his constructs is also physically exhausting as he creates open wounds which heal closed thanks to the minor healing properties that come as complimentary ability to his quirk. This healing puts a minor strain on his body but repeated uses stack up. Lastly his quirk causes him pain when it breaks the skin, he's acquired a higher pain tolerance because of this but pain is still pain.

Hero Outfit: The tech aspects of Max's "Skeleton Suit" include a HUD with lie detecting software, translator software, and facial recognition software. He also has a utility belt with calcium supplements, cuff tape, batteries, a burner phone, and smoke bombs. He also has a watch that monitors his biological functions, dietary statistics, injury locations, and other health based statistics. He also carries a laptop for hacking into villain systems, and medical supplies in his small backpack. Lastly for weapons he carries two stun batons and a non-lethal taser pistol.

https://preview.redd.it/0u49z2itnyo51.jpg?width=1000&format=pjpg&auto=webp&v=enabled&s=071bd8243906beb5458d1b0e56e0c66b9b039891

Personality: Max is a lovable geek who's a lot like Deku in terms of his fanboy status. He's big into pop culture media, and takes a lot of inspiration from Spider-man. He likes to joke in combat to distract his enemies, he makes references and likes to point out cliches. He's highly intelligent, making most his hero gear with some help from friends. He's not agaisnt killing but avoids it at all costs, usually giving villains a warning that if they try escaping and hurting people again he won't be as kind. Still he tries to uphold the law as best as possible, usually carrying a body cam of some sort that he gives to police for inspection to make paperwork a little easier on them. All in all he's the kind of hero who does his best but won't hesitate to do what needs to be done for the greater good.

Backstory: When Max was born his parents ended up having to leave him with the hospital due to learning that their son had a degenerative bone disease. They couldn't afford to take care of him so he was left an orphan. So Max grew up in the hospital mostly alone, reading books to occupy himself. This is where his intellect grew and he grew into quite the inquisitive boy. After 13 years of being hospitalized his quirk finally manifested. He was a very late bloomer but when his quirk did manifest his disease was able to be mitigated by his ability to grow and reinforce new bone matter. Granted it took him a few months to learn how to use his quirk to heal his body but after that he was approached by some small time american heros who suggested he use that power to be a pro hero. Seeing that he could help others avoid life in a hospital like him he agreed and enrolled in a basic america school for heros.

Ultimate Move: Shatter Spine - Max creates a hyper dense super compressed spinal cord arm spear that decompresses a moment before impact causing it to act like a railgun shot shattering whatever it hits. This takes him a few hours to construct the blueprint for in his body, basically he reserves resources and imprints a specific build blueprint into it. Upon use it crumbles from the force and can't be used till he spends a few more hours repeating the process.