It seems as if Paul is climbing the rungs of authority to hear his case without even attempting to gather attention. It reminds me of a case that goes before our courts and then eventually ends up in the Supreme Court. Paul had become, by this time, quite a celebrity. Felix was no longer the procurator of Judea, but he has been replaced. The way that you can think of it in modern terms, is that Judea could be a sort of state within the Roman Empire. Festus was the new governor. Agrippa, who is called King in this Scripture, would be the next person in power just under the Emperor. He took the charge from Herod. Agrippa has an interesting history. Bernice, who is mentioned as accompanying Agrippa, is actually Berenice and is the sister of Agrippa. The gossip of the day was that he was living in an incestuous relationship with her. There was much tension between Jerusalem and the Jews living there and Rome. Agrippa had just built a palace that blocked the view of the temple. They ended up kicking him out in 66 AD, which would have been soon after this trial of Paul.
Notice that throughout these verses in chapter 25 the high priest and his cronies are wanting to get Paul back to Jerusalem because they knew that once he was back there they would be able to dictate what would happen to him. They had control over Jerusalem much more than the Romans did. But Paul did everything in his power, and he succeeded, in getting himself an audience with the Emperor which would prevent him from going to Jerusalem and give him at least half a chance of saving his life. Even Festus said he saw nothing wrong with Paul. That should sound vaguely familiar to Jesus and Pontius Pilate who said he saw nothing wrong with Jesus.
The audience that Paul has with Agrippa and Berenice is a big deal. The next chapter lays out what Paul says to them. I think you’ll like how he leads them to a knowledge of Jesus.