Luke

Luke the Evangelist—also known as Luke the Physician and Saint Luke—is the traditional author of both the Gospel of Luke and the Book of Acts. He wasn’t one of the Twelve, and most scholars wouldn’t consider him an apostle.

Post-Gospels
Luke appears to have been a companion of Paul. At times in Acts, he includes himself in the story:

And while Luke didn’t personally witness Jesus’ ministry, he claims to have “carefully investigated everything from the beginning” (Luke 1:1–4) and he had access to eyewitness accounts, including the Gospel of Mark.

In the fourth century, a bishop claimed Luke was one of the Seventy (or Seventy-Two) disciples mentioned in Luke 10, but that’s unlikely, especially since Luke appears to indicate he was not an eyewitness in Luke 1:1–4.

While he was certainly an important part of the early Christian church, Luke was not one of the 12 apostles.

Authorship
Traditionally wrote both Luke and Acts.