In the context of the original Harry Potter story, it seems quite abnormal as Umbridge is a very unlikable character to Harry. But in the world of fanfic, anything can happen. Fanfic allows for different interpretations and character developments, so while it's not normal in the traditional sense, it's possible within the creative space of fan - created stories.
This could imply a lot of things. Fanfic is a realm of creativity where the normal boundaries of the original story can be bent. Harry liking Umbridge fanfic could be a sign of his growing maturity in the sense that he is willing to explore different perspectives, even of someone he dislikes in the original story. It might also be that the fanfic has a unique take on Umbridge, perhaps making her a more sympathetic character, and Harry is attracted to this new interpretation.
In certain fanfictions, Harry could get out of trouble with Umbridge through the power of truth - telling spells. He might be able to prove his innocence by using a spell that forces the truth to be revealed. Also, there could be instances where magical creatures come to his aid. For example, if he has befriended some of the magical creatures on the grounds, they could cause a commotion that diverts Umbridge's attention long enough for Harry to get away. Maybe a group of pixies start causing chaos in her office, and while she's trying to deal with them, Harry slips away unnoticed.
One possibility is that Umbridge, despite her cruel nature, sees some potential in Harry for her own gain. Maybe she tries to mold him into a perfect little wizard according to her strict and often wrong ideas. She could isolate Harry from his friends and force him to follow her rules blindly, which would create a lot of internal conflict for Harry as he tries to resist her brainwashing while living under her roof.
Well, if Umbridge uses Veritaserum on Harry in a fanfic, it depends on how the author writes it. Some fanfic writers might have Harry spill out information about the Order of the Phoenix, which could put the whole organization at risk. Or perhaps he'd talk about his experiences with Voldemort that he's been keeping to himself. This could potentially change the whole dynamic between the characters in the fanfic, especially if Umbridge uses this information to her advantage.
Well, it could be that Harry gets tired of Umbridge's tyrannical rule at Hogwarts. He might start by secretly organizing the students to resist her unfair rules. Maybe he uses his status as the Boy Who Lived to inspire others. For example, he could encourage students to defy her ban on certain magical activities. Then, he might start a campaign of small acts of rebellion, like writing anonymous letters about the truth of what's happening in the school.
Some of the older students like the Gryffindor prefects could support Harry. They understand the importance of a just Hogwarts environment. Neville Longbottom could also be a strong supporter. He has grown a lot in courage throughout the series, and this would be his chance to show his loyalty to Harry and fight against Umbridge's unfair treatment of the students.
The development of their relationship in Harry and Umbridge lemon fanfiction can be quite diverse. It could be that Umbridge, in an attempt to control Harry more closely, gets drawn into his world in a way she didn't expect. Harry, on the other hand, might start to see through her tough exterior and find some vulnerability in her. This discovery could lead to a series of exchanges between them, some filled with tension, some with a strange sort of understanding, and over time, this shapes their relationship in the fanfiction.
One major consequence could be that Harry reveals his connection to Sirius Black. If Umbridge gets to know about this through the Veritaserum, she could use it as a weapon against Harry. She might try to capture Sirius or use his existence as a way to blackmail Harry into following her every order. Also, Harry could end up revealing the details of how he has been learning magic outside of the school curriculum, which would be a huge violation in Umbridge's eyes.
Harry's character would face a huge transformation. He'd be constantly on edge, having to deal with Umbridge's strict rules and her mean - spirited nature. This would make him more empathetic towards those who are oppressed. He'd also become more resourceful, finding ways to communicate with his old friends and allies despite Umbridge's attempts to isolate him. His experience with Umbridge would be a crucible that forges him into an even more resilient and determined hero, ready to take on not only Voldemort but also the corrupt elements within the wizarding establishment.