I somehow didn't add Morte into my party at the start of the game. I didn't realize he could be a party member. (I went in blind to this game)
Anyway, is there a way to go back and get him?

If not, how much will he be missed later in the game?

EDIT:
Nevermind. I died for the first time and when I appeared back in the mortuary, he was there. I clicked him and he joined the party.