Chronologically, Days is the second game because it starts a little before CoM does. In terms of release, Chain of Memories is as it was released on GameBoy Advance in between KHI and KHII. As far as main games, KHII is the second game.
This is a (intentionally somewhat late) Christmas present for the ever-awesome Rebmakash! She mentioned that she wanted to see some sort of crossover between Kingdom Hearts and Undertale, and it would.