Silvanus? Who is that guy?
Silvanus is another name, or perhaps a translation of, Silas. Silas, of course, is the one that spent a night with Paul in prison in Acts 16. Silas is also mentioned in several of Paul's letters under the name Silvanus (2 Corinthians 1:19; 1 Thessalonians 1:1; 2 Thessalonians 1:1).
Peter also mentions him once:
Through Silvanus, our faithful brother (for so I regard him), I have written to you briefly, exhorting and testifying that this is the true grace of God. Stand firm in it!1 Peter 5:12
Why does Peter mention that he regards him as a faithful brother? Isn't that kind of obvious from our perspective?
Well, to answer this, we have to ask who Peter's audience was. Without going into much detail, Peter, being the apostle to the circumcision, was writing to the Jewish diaspora (1 Peter 1:1). This detail reveals why Peter would need to clarify why he was working with Silas.
It all goes back to Peter's connection with the Jerusalem church as an elder and apostle. Even though Peter was the first to preach the gospel to the Gentiles, he treated the Gentiles like second rate Christians and caused several others to do the same by his example. It was for this reason that Paul had to stand up to him in front of everyone:
But when Cephas came to Antioch, I opposed him to his face, because he stood condemned. For prior to the coming of certain men from James, he used to eat with the Gentiles; but when they came, he began to withdraw and hold himself aloof, fearing the party of the circumcision. The rest of the Jews joined him in hypocrisy, with the result that even Barnabas was carried away by their hypocrisy.Galatians 2:11–13
Because of this contention between Peter and Paul, there arose several factions within the church. There were even divisions on whether or not Paul was nullifying the law with his teachings, an accusation he denied (Acts 21:20-21).
Now I exhort you, brethren, by the name of our Lord Jesus Christ, that you all agree and that there be no divisions among you, but that you be made complete in the same mind and in the same judgment. For I have been informed concerning you, my brethren, by Chloe's people, that there are quarrels among you. Now I mean this, that each one of you is saying, “I am of Paul,” and “I of Apollos,” and “I of Cephas,” and “I of Christ.” Has Christ been divided? Paul was not crucified for you, was he? Or were you baptized in the name of Paul?1 Corinthians 1:10–13
Paul condemned these factions, and he affirmed Peter's apostleship and witness to the resurrection (1 Corinthians 15:11).
So, when Peter mentions Silas towards the end of his letter and comments on Silas' faithfulness, he is demonstrating to his Jewish audience that there is no division between the two ministries. In Peter's second letter, he does the same thing except he takes it a step further and mentions Paul directly:
Therefore, beloved, since you look for these things, be diligent to be found by Him in peace, spotless and blameless, and regard the patience of our Lord as salvation; just as also our beloved brother Paul, according to the wisdom given him, wrote to you, as also in all his letters, speaking in them of these things, in which are some things hard to understand, which the untaught and unstable distort, as they do also the rest of the Scriptures, to their own destruction.2 Peter 3:14–16