Goku would react with pure determination. He'd use all of his skills and powers to track Raditz. His love for Gohan is so strong that he'd even consider pushing his own limits, like going Super Saiyan if he could at that point. He'd be focused solely on getting Gohan back from Raditz's clutches.
Goku would be a mix of angry and worried. His Saiyan blood would make him want to rush into battle with Raditz right away. But he also knows how dangerous Raditz is, so he might first try to gather some allies, like Piccolo. He'd be constantly thinking about Gohan and how to rescue him as quickly as possible.
One plot idea could be that Gohan's friends, like Krillin and Bulma, team up to rescue him. They might use some of Bulma's inventions to track Raditz. Another idea is that Gohan manages to outwit Raditz during the kidnapping. Maybe he uses his hidden power to escape Raditz's clutches briefly and leaves a trail for his father Goku to follow.
They'd be shocked. Goku is a legend in the Dragon Ball world, so they'd be super excited and probably a bit scared too.
Piccolo might also be a savior. He has a soft spot for Gohan since they trained together. He could use his strategic mind and powerful attacks to outwit Raditz and rescue Gohan. Maybe he teams up with Goku for an even more epic rescue mission.
Goku and Grandpa Gohan's interaction could be full of wisdom - sharing. Grandpa Gohan was a wise man, and he would pass on more of his knowledge to Goku. Goku, in turn, would tell Grandpa Gohan about all the new friends he has made and the different planets he has visited. They might also spar a bit, just like in the old days, with Grandpa Gohan giving Goku some pointers.
Yusuke would be extremely angry. He would immediately start looking for Keiko. Yusuke is very protective of her, so he would drop whatever he was doing and go on a full - scale search. He might first try to find Hiei's usual hiding places in the demon world and question any demons he comes across.
Christine might initially be very frightened. After all, being kidnapped is a terrifying experience. She could scream and try to fight back.
In some cases, the reader could be in a state of shock and disbelief. They might not believe that this is really happening. But as they come to terms with the situation, they could start to form a plan. Maybe they try to appeal to Loki's better nature, if they believe he has one, by talking about his past actions and relationships that show his capacity for good.
At the start, Ichigo would be defiant. He would not show any fear in front of Aizen. He might taunt Aizen, trying to get him to reveal his true intentions. As time goes on, he would start to feel a sense of isolation, but this would only strengthen his resolve. He would use his inner Hollow powers more freely, as he has no other option. He would also be constantly thinking about ways to turn the tables on Aizen and escape, and would be ready to take on any challenges that come his way during his captivity.
Naruto would be extremely enraged. He would vow to rescue Sakura no matter what. He would immediately start gathering his friends and form a rescue team. Sasuke, on the other hand, might initially seem more reserved but would also be determined to get Sakura back. He would use his Sharingan skills to track Madara.
Lydia would probably be very scared at first. After all, being kidnapped by a strange and unpredictable character like Beetlejuice is no joke.