Izuku Midoriya has this in spades, which goes hand-in-hand with his big-eyed, baby-faced design and boyish voice. He is extremely shy around girls, turns beet-red almost every time Uraraka is close to him, mumbles endlessly about his favorite Heroes and is very chipper about everything he does. He might as well be anime's Trope Codifier for this!
Ochaco Uraraka herself is adorkable whenever her crush on Izuku is brought up. One anime-exclusive scene has her very physically close to her friend when excitedly bringing up the training camp, and gets confused when she notices him turning beet red. When she notices how physically close she was, she gets an adorable Crush Blush of her own and distracts herself, and a couple of her classmates, with a song and dance number.
Even All Might, despite being the Symbol of Peace and an imposing man, manages to be adorkable at times thanks to the sheer enthusiasm he has about being a hero. He once made a bombastic entrance that his protégé Midoriya was in the vicinity, right before offering to eat lunch with him; even Uraraka lampshaded how adorable that was.
Tenya Ida is a very earnest young man, so dedicated to being a hero that he gives 100% of his determination into everything he's tasked with, even playing a villain or simply dancing on stage.
Shoto Todoroki: he tends to be stoic and reserved, but that doesn't hide how dorky he is - he genuinely wonders if he's cursed when it turns out two of his classmates hurt their arms when he was around, metaphors completely go over his head and he has very bad timing when he wants to say something. Couple that with him being a Chick Magnet both in-universe and meta, and it serves to make his attempts at resocializing more endearing.
Eijiro Kirishima is absolutely obsessed with being "manly" and desires to be just like his idol Crimson Riot, however his obsession manages to be rather endearing and adorable than anything else and he's even shown to be a bit insecure whenever someone scoffs at his obsession.
Momo Yaoyorozu can be a bit too enthusiastic about a lot of stuff when she was taught to be a proper lady. She claims she has serious reasons for acting the way she does, but everyone else can see that she's got the heart of a curious and eager little girl wanting to act like a normal girl would.
Kyoka Jiro is normally very forward, but bring up the subject of her personal and innermost desires, usually music, and her adorable shy streak emerges. She even engages in Shy Finger-Twiddling with her earlobes when flustered.
Fumikage Tokoyami has a tendency to say dramatic things about situations that don't quite deserve them, such as calling a recreational game on their school trip a "mad banquet of darkness", or giving his attacks overly dramatic and dark-sounding names. Even his room's decor is dark and spooky, and he got angry when no one took it seriously.
Koji Koda becomes this later on, in no small part due to his shyness and love for animals which caused him to bring a pet bunny to his dorm room.
Inko Midoriya, Izuku's mother, is just as adorkable as her son. She's timid, easily startled and prone to overreacting, but also a doting mother who is constantly worrying about the safety of her son, panicking to the point that she can barely get across the floor on all fours, pacing back in forth in the hallways, and crying so much that she knocks herself unconscious from dehydration.
Shihai Kuroiro has shades of this when interacting with his crush, Kinoko Komori. He starts to stutter whenever he talks to her and is internally overjoyed when she compliments him during a training match.
Inasa Yoarashi is extremely loud and hot-blooded, but that doesn't stop him from being extremely enthusiastic and upbeat about everything around him, acting overly friendly to almost everyone, with the exception of Shoto and Endeavor (at least at first).
