How did he get here?

The Mimic comes in Ruin, Mainly a Universe where he manages to Kill Cassie. After these events he would be free from the Pizzaplex and would terrorise people. Then the Wormhole came, pulled him in, and he's here

What are his Personal Relationship with others?

Rewrite: A foolish Jester. Purely some junk.

Withered Foxy and Bonnie: He loves to Mimic the Voice of William Afton around them, just to get them roudy and Pissed.

Doll: No opinion

Theodore Peterson: Barely Even bats an eye on him

Cyn: same as Theodore Peterson