So, for merged cards (Fusions), it appears that the new card uses the primary (first) ability score of the item and character, and then 80% of the sum of the attack and HP (rounded down). But I cannot figure out what directly affects the size of the new primary ability, though what the ability is will always be determined by the item card. Rarity seems to have the largest affect on size, but there has to be something else...

Another random thing I noted is that there seems to be item sets (2 golden condom sets, 1 of each other new primary ability set for items currently in the gacha [There are a couple cards like the library and university that I cannot verify]). Each character is assigned 1 or more sets, which means if any card from a set is assigned to that character, all cards from the set are assigned to that character. (For Fusions...)

I don't know if any of this will be useful to anyone but there is that.