He may react with sadness and withdrawal. Instead of getting angry, he could just retreat into himself, crying alone and avoiding social interactions. This could be a sign of his deep - seated hurt and disappointment.
Issei could react with anger. He might lash out at his partner and the person they cheated with. He could also become very confrontational, demanding explanations and apologies.
To determine if an 'issei' has cheated on fanfiction, we would need to look at the specific fanfiction in question. Different fanfictions have different storylines and character developments. Maybe in some very niche or specific fanfiction, there could be a situation where an issei - like character is involved in cheating, but it's not a common or known occurrence in general fanfiction.
One possible plot could be that Issei discovers his partner cheating through finding love letters or messages from the other person. He then goes through a phase of heartbreak and self - doubt, questioning what he did wrong.
At first, he's likely to be shocked. Issei is often portrayed as a character who trusts easily, so betrayal would come as a big surprise. He might not believe it at first and try to find reasons to justify the actions of the person who betrayed him.
Issei could develop by learning the discipline and self - control that comes with being an exorcist. This would contrast with his more impulsive devil - side nature. For example, he might have to resist the urge to use brute force and instead use the more strategic and spiritual techniques of an exorcist.
Issei would most likely become more protective. He'd be constantly by Rias' side, making sure she was comfortable and safe. He might also start training even harder to ensure he could defend his family when the baby arrives. His character would grow in the sense that he'd become more selfless and family - oriented, thinking about the future of his family rather than just his own growth as a fighter.
In some fanfictions, Issei's character develops by becoming more mature. He realizes the consequences of his actions more deeply. For example, when he uses his powers, he starts to consider the collateral damage and tries to minimize it, which is a big step from his more impulsive self in the original story.
Rias would be initially cautious. Since she was so used to Issei, the new OC would be an unknown factor. She would likely test the OC's loyalty and abilities. Akeno might be intrigued by the change, seeing it as an opportunity to get to know someone new. Koneko, on the other hand, would be more reserved and take her time to warm up to the OC.
It depends on different perspectives. From a purist point of view, it's likely to be seen as inappropriate because it may distort the original intention of the source material. But for those who view fanfiction as a space for unrestricted creativity, it could be seen as an interesting exploration. However, it should be noted that if the work is shared publicly, it may receive a lot of criticism due to the sensitive nature of the topic.
I haven't actually read a specific fanfiction with this plot. However, if such a fanfiction exists, it would likely involve a lot of creativity in terms of how the relationship between Issei and Serafall developed to that point. It could explore different aspects of their personalities and how they would handle the situation of a pregnancy, which might be quite different from the original storylines of their respective characters.
Issei develops by facing various challenges. Each encounter in the harem and with the milf - hunting situations makes him grow. He becomes more confident over time.