Active Bond
Girl got bullied for most of her life, never having friends and neglected by family members and others alike due to being perceive as quirkless. These only worsened as an injury caused by one of her bully's quirk, injure her severely.
Noticing the reaction being worse then before, the bullies ran away in fear of being caught, while the people around her were either just watching hoping that a hero would come and save the day or quickly walk away not wanting to be there.
The injury dealt to her was a lot worse than expected. The heavy blood loss & the many physical injuries cause her to slowly lose consciousness. Due to imminent death her hidden quirk awakens, as a semi-sentient entity emerges from her body, using her blood as a medium. such display frightens the citizens.
The police, receiving a call by a scared civilian saying that a girl is being attack. Arriving to the scene, the policemen watch as people screaming and running away from some sort of red liquid monster. (Blood ya can't think of a name).
Mistaking 'Monster' for killing the girl, heros were dispatched to 'stop' the monster, aka Endeavor was nearby so he was sent to destroy it. Not caring Endeavor pushed past the police and...
...
...
Does whatever he wants.
Meaning attack first questions later. Which by far was the stupidest move anyone in the vicinity had seen. Being attacked, 'Blood' now sees everything as a threat to its host and starts lashing out, creating chaos to the surrounding.
Surprising the heros by not even being fazed by Endeavor's attack. 'Blood' attacks back with its elongated arms which have turned into various sharp and blunt weapons.
(Cannot be destroyed physically but its condition will vary due to its host emotion & condition.)
Not even 5 minutes had passed, All Might, Eraser Head arrived after receiving a call from a policeman to see most of the heros on the floor unconscious and injured. Endeavor being the one who is still awake and fighting 'Blood' has received the most damage.
"That girl is still in there." The number 2 hero grunted still fighting the bloody creature. Attention drawn away from the fight, both notice the huge red blob of blood that seemingly looks alive.
Attempting to aid EraserHead tries to erase the creature's quirk but to his shock couldn't. All Might being All Might punch the blob revealing the girl within for a second before being engulfed back like nothing happened to the shock of both the number 1 and underground heros.
A plan by Eraser Head which involved All Might punching the blob revealing the girl and Eraser stopping her quirk, which in turn causes 'Blood' to stagger and collapse into a pool of blood.
(Either hitting it so hard that it cannot take any more blood to protect its host/regenerate/attacking the host or have Easerhead doing whatever he does best.)