There are several Trump surrogates who are under investigation in the Russia investigation. Trump's campaign has tried to distance them from any important role in the campaign, but Trump has not said much about them. Michael Flynn is the exception. Trump asked the FBI director Jim Comey if he could do something to contain the Flynn investigation. He claimed that Flynn was a good guy. Perhaps a better reason for Trump's efforts on Flynn's behalf is that he had important contacts with Russian intelligence agencies and with cybersecurity firms that could easily have been used in Russian hacking attacks. For example, Flynn had relationships with Kaspersky Lab which has expertise in cybersecurity as well as relationships with Russian security organizations. Flynn also had connections with GRU which is the Russian military intelligence organization. Nobody in the Trump campaign would have been better equipped to collude with Russia than Flynn.
Flynn argued that he cultivated his relationships with Russia during the campaign to develop Russia as an ally in US efforts to defeat Islamist militants. Trump made the same argument. Perhaps there were other reasons for allying with Russia that were more germane to the Trump campaign.
Trump was forced to fire Flynn soon after his election. He claimed that it was because Flynn lied to Vice President Pence about contacts he had with the Russian ambassador. That explanation does not pass the smell test. Lying to Pence is not the kind of thing that would bother Donald Trump. The more reasonable explanation is that Trump knew why Flynn may become a liability to him personally. It took 18 days after Flynn lied to Pence for Trump to fire him. He may have used that time to work things out with Flynn if he believed that the Flynn investigation might lead back to him. That would be a good reason for Trump to have exposed himself to risk by attempting to influence the Flynn investigation. If there was collusion with Russia during the Trump campaign, Michael Flynn would have been at the center of it. He was in the cybersecurity business as a consultant. He was also connected to expertise in the US and in Russia, along with his connections to Russian intelligence agencies. Flynn offered to testify in the Russia investigation if he were granted immunity. He must have reason for requesting immunity. There is also a good reason why his request from immunity was not granted. Investigators must expect that the case against Flynn will be further developed. The more severe the charges against Flynn, the more likely he can be flipped. That may be what keeps Trump awake at night during his Twitter splurges.