介 drew a person wearing armor plates front and back — later read as a person standing between two sides. Everything 介 does happens in the middle: introducing two strangers, mediating, or being the interface (界面's cousin).
自我介绍 self-introduction is the ritual opening of every Chinese classroom, job interview, and blind date — name, hometown, hobby, in that order. 介意 'to mind' asks whether something crosses your boundary: 你介意吗？
A person 人 standing BETWEEN two lines — the INTRODUCER in the middle.
简介 'brief introduction' is the word on every app store page and book jacket — including the one you read before downloading this app.