Midoriya starts out as a timid, emotional, courageous and altruistic boy. He initially had difficulty asserting himself and believing in himself and acknowledging his accomplishment. As time passes he grows out of this and gains confidence in his abilities and becomes able to properly express himself to others.
One of Midoriya's most defining traits is his analytical nature particularly towards heroes and the quirks of others. He keeps notes on pro heroes and their quirks. Due to this he possesses extensive knowledge on pro heroes and knows how to spot weaknesses in quirks and how to use the abilities of others and himself to formulate effective strategies.
One For All
Izuku's quirk is a power stockpiling quirk passed down for generations through multiple users. He is the 9th holder if this ability known as One For All. This quirk grants him high levels of strength, speed and durability. A strong body is required to use it at full power and due to him not being strong enough he can only safely wield a certain percentage of his power. This limit is currently 45%.
Due to One For All's origin it holds the amplified abilities of the past users. Currently Izuku can use 2 of the 6 abilities within his quirk. These are:
Blackwhip: This quirk belonged to the fifth user and allows Izuku to create black tendrils of energy from his hands.
Float: This quirk is from the seventh user and allows Izuku to float in mid-air.