Learn Finnish: Se on mun uusi kämppis.

In written standard Finnish, se means "it" and hän means "he" or "she". However, in everyday spoken Finnish (puhekieli), people almost always use se to refer to other people. So in this context, se naturally translates to "he" (or "she").

Since se is a spoken-language replacement for hän, does it change how the verb on is conjugated?

No, both se and hän take the exact same third-person singular verb. Whether you write hän on in formal text or say se on in conversation, the verb form stays exactly the same.

Where does the word kämppis come from?

It is a very common spoken-language word for "roommate". It comes from kämppä (a colloquial word for an apartment or pad) combined with the -is suffix, which is often used to create slang words in Finnish (like kirppis for flea market). A more formal equivalent would be kämppäkaveri or huonetoveri.

I see mun here instead of minun. Do I always have to use it with spoken-language words?