Well, obviously Deku in two forms - the younger and the older version are the main characters. The younger Deku is the one who is still growing and learning, while the older Deku has more experience.
Another plot might involve the older Deku being from an alternate timeline. He shows the younger Deku different ways he could have developed his skills and relationships, which makes the younger Deku reevaluate his own path as a hero.
As a child, Deku would likely be more naive. He might not fully understand the extent of the challenges that lie ahead in his journey to become a hero. His admiration for heroes would be pure and untainted. Uraraka as a child could be more impulsive. For example, she might use her quirk without thinking about the consequences as much as she would when she's older. She'd also be more likely to show her emotions openly, like her excitement about hero - related things.
The main differences are in their expressions and some minor details of personality. In the manga, Deku might seem a bit more intense, while in the anime, he's sometimes presented a bit softer.
As of now, Deku is very much alive in the manga. The plot hasn't reached a point where he meets such a fate. Usually, main characters like Deku have significant story arcs and aren't easily killed off.
He might cry a lot. Deku is very emotional, and being cheated on would be a very painful experience for him. He could also get angry, which is rare for him, but this situation would be so unjust that he might not be able to hold it in. He'd probably start to question his self - worth too, thinking that there was something wrong with him that made his partner cheat.
Deku might be initially shocked. He's used to Bakugou's aggression, but an actual injury is different. He could be in pain and disbelief at first. Then, he might try to downplay it, especially if he doesn't want to cause trouble for Bakugou, because Deku is generally a very forgiving character.
In the manga, Deku experiences various ups and downs. He not only becomes stronger physically but also matures emotionally as he takes on greater responsibilities. There are also many unexpected plot twists that shape his journey.