Izuku might be shocked at first. After all, he has grown up without his father. But then he could be filled with a mix of emotions like joy, confusion, and anger all at once.
When Izuku meets his father in the fanfiction, his reaction could be very complex. He may initially be in disbelief and think it's some kind of trick. Once he realizes it's really his father, he could be overjoyed and start asking a million questions about why he left and what he has been doing all these years. He might also be a bit angry for being abandoned for so long, but overall, the excitement of finally having his father back could overshadow the negative emotions.
In the fanfiction, Izuku's reaction to meeting his father would depend on how the author has set up the story. If the father is presented as a hero figure, Izuku could be in awe and look up to him even more. He would be eager to learn from his father and bond with him. However, if the father has a more ambiguous or negative past, Izuku could be hesitant and cautious. He would still be curious about his father but also on guard, not knowing what to expect from this new relationship.