This isn't a run-on sentence, but it's verbose and not idiomatic. It's also not in a consistent register. The first part (which I deleted and rewrote) is a pretentious attempt at being very formal, but the rest is colloquial. It could be:
We're pleased to inform you that Rahul, a new housemate, will be moving in this week. He seems to be a very calm guy: staid and easygoing.
Yes, it should use a colon instead of a semicolon, or it could use a comma or a dash of some kind or parentheses for those last two words.