Onesimus - the slave mentioned in the Epistle to Philemon (Phm 1:10, Phm 1:16), “a brother beloved.”
a faithful ... brother - rather, “the faithful brother,” he being known to the Colossians as the slave of Philemon, their fellow townsman and fellow Christian.
one of you - belonging to your city.
They shall make known unto you all things - Greek, “all the things here.” This substantial repetition of “all my state shall Tychicus declare unto you,” strongly favors the reading of English Version in Col 4:8, “that he might (may) know your state,” as it is unlikely the same thing should be stated thrice.