Cactus Game Design Message Boards

No. Must declare before battle.

You have to choose the Lost Soul before battle, but what if I choose a Lost Soul that I either know or hope they have in their deck and search for it to bring it out (or they happen to draw it, or something)? Does the target of your choosing have to be in play at the beginning of the battle phase?

While not explicitly stated, the target does have to be in play at the beginning of the battle phase. There is a line about the chosen LS being removed and returned (specifically shuffle and discard, but should apply to topdeck/underdeck as well) not keeping the targeting, so even if you could choose a LS not in play, it would immediately reset to face value (not being chosen) while in deck or discard pile.