I'm trying to do a quest where you have to get into this building that is locked and my lock picking skill isn't up far enough, so I'm curious if there's any companions that can pick locks.

cait... cait can pick even master locks you find her after clearing out the combat zone.

There is usually an alternative way in - which quest is it?

It's called public knowledge, I have to return a book to the Boston public library.

I looked around and couldn't seem to find an alternative way in or a key.

there are 2 ways in lol... neither of them need lockpicking.. you can use charisma to convince security on the front door to let you in.. or.. you can go in thru the subway.

but if you really need to pick a lock.. get cait she can pick any lock.

I went into the subway and the only door I could find to the library was also locked. I don't see a guard anywhere either. I guess I'll just wait until I get cait.

next to the door on the outside is a small intercom. Walked past it a couple times