The main protagonist of My Hero Academia, Izuku Midoriya -- otherwise known as Deku -- is known for his reckless nature. He is introduced as a nervous and Quirkless kid with a firm hold on a dream of becoming a Pro Hero that seems out of reach. That is, until he meets All Might, the Number One hero and Symbol of Peace, who selects Izuku to be his successor after being encouraged by him and witnessing his tenacity and kind nature in action.
Trying to navigate his newly-gained Quirk is a quest in and of itself and Izuku quickly finds trouble from the moment he enters the prestigious U.A. High School, with countless villain encounters and many more injuries. Slowly, he begins to get a handle on his unstable Quirk, but his body retains (and acquires) some quirky aspects of its own. Here are five of the weirdest.
Green Lightning
Izuku goes through a number of stages as he figures out different ways to use his Quirk without shattering his bones. When training with Gran Torino in Season 2, he learns to activate a small percentage of One for All throughout his entire body, which he calls Full Cowling. In this state, his body fizzles with green lightning, which is enhanced depending on the amount of power he exhibits. Apart from green lightning, his Quirk is also visualized with matching glowing eyes and striking red veins.
One for All is the cultivation of raw power used to enhance the user's existing abilities to a superhuman level. Neither All Might nor Nana Shimura, two predecessors of this Quirk, were ever shown to radiate any visual effects when using it. Some fans have theorized that the green lightning is either excess kinetic energy or One For All's aura unique to Izuku, himself. We may not have an explanation for the green lighting, but at the very least, it looks really cool.
Using Broken Bones
As Izuku's body grows and adjusts to his Quirk, he experiences a lot of trial and error, ending with many broken bones. Normally, such a severe impediment would end a fight; however, in Izuku's case, he's able to keep going under extreme pain. During the Sports Festival, he used his already broken bones to launch more attacks.
In these instances, Izuku experiences an adrenaline rush through being confronted with the possibility of failure, which numbs the intense pain somewhat and helps him power through. There have been longer-lasting physical repercussions as a consequence, but the ability to still use his Quirk and fight with extreme injuries is an amazing, if not terrifying, feat.
1,000,000 Percent
Izuku is still unable to use 100 percent of One For All's power without injuring himself so far in the anime. During the Training Camp arc in Season 3, he's forced in a battle against Muscular to save the life of Kota. Similar to how he pushes himself when his bones are broken, he reaches 1,000,000 percent of his power to defeat Muscular. If 100 percent shatters his bones, how then, can Izuku use 1,000,000 percent?
Although it is never explained, he probably didn't actually exhibit that much power when he could only safely handle five to 10 percent at the time. After being overwhelmed by Muscular's strength and tearfully thinking of his mom and All Might, it is more likely that he simply said he was using that amount to hype himself up, using his school's motto of "Go beyond! Plus Ultra!" to surpass his limits before claiming his victory.
Muttering
An interesting oddity that Izuku has is his muttering. As an avid fan of heroes and Quirks, he gets excited easily in the presence of a hero, or when witnessing new Quirk. While taking notes on these things, his passion causes him to be scarily enthusiastic, causing him to mutter all of his thoughts out loud.
So absorbed is he in his internal analysis that he is hardly aware of this habit when it occurs, but the people around him certainly are. They are almost always confused or concerned by it, staring in shock as a storm of mumbles surrounds them, especially when it happens unexpectedly. Having seen the dumbfounding effect it has on his peers, it might be practical for Izuku to use this habit to confound an opponent during battle.
Intense Crying
Due to his past struggles, Izuku grew up as an extremely nervous crybaby -- traits we've seen from his mother, too. It's not unusual to see him cry his eyes out in stressful situations, especially in the first few seasons where it's a near-constant thing. This stems from him being looked down upon, making him insecure, a trait that is evident in Season 1.
But what started as a hangover from his difficult childhood has evolved into exaggerated reactions that are now used for comedic effect. It's reached a point where his eyes spout literal waterfalls making him the human equivalent of a fire hydrant. In Season 2, at the end of the Cavalry Battle, his intense crying even broke through the ground at his feet. Maybe intense crying was his real Quirk all along...