Website and data research by 101Leafy.

Fusion in Megami Tensei Gaiden Last Bible for the Game Boy Color is the spell Combine, cast from the map menu. 101Leafy researched the game and tried the rules out in play. The calculator uses each monster's starting level, the level it has when it joins.

Rules

1. Cast Combine from the map menu and mark two monsters. The spell costs 5 MP, paid only when the fusion goes through.
2. Every monster belongs to one of 11 races, and the chart gives the race of the result for each pair. Monsters of races 9, 10, and 11 can't be put into a fusion.
3. Two monsters of the same race always make Lich, with no hero level check.
4. Otherwise the game takes the average of the two levels, rounded down, and picks the first monster on the result race's list whose fusion level is at least that average. If none is that high, it takes the last one on the list.
5. Slime and Lich (race 8) count as their partner's level plus 1.
6. The result's fusion level can be at most the hero's level plus 2. Above that the game refuses the fusion.
7. A monster that is already in the party can't be made. That includes the two being fused, so a pair whose result is one of them fails.
8. Where the chart has a *, the fusion has a 10% chance of a mutation if the monster of the row's race is higher in the party list. The result is then the first monster on the mutation list whose fusion level is at least the hero's level plus 3, as long as that one is not already in the party.
9. After the Orbs are retrieved, the special fusions come first. They skip the chart and the hero level check.
10. Both monsters leave the party. The new one joins at its starting level with its starting stats.

Race chart

Rows are the monster higher in the party list, columns the lower one, and each cell is the race of the result. * marks a 10% chance of a mutation.

	Race 1	Race 2	Race 3	Race 4	Race 5	Race 6	Race 7	Race 8
Race 1	Lich	Race 7 *	Race 5	Race 2	Race 4	Race 3	Race 6	Race 1
Race 2	Race 7 *	Lich	Race 5	Race 1	Race 4	Race 3	Race 6	Race 2
Race 3	Race 5	Race 5	Lich	Race 2	Race 5	Race 7	Race 6	Race 3
Race 4	Race 2	Race 1	Race 2	Lich	Race 2	Race 5	Race 2 *	Race 4
Race 5	Race 4	Race 4	Race 5	Race 2	Lich	Race 4	Race 6	Race 5
Race 6	Race 3	Race 3	Race 7	Race 5	Race 4	Lich	Race 3	Race 6
Race 7	Race 6 *	Race 6	Race 6	Race 2 *	Race 6	Race 3	Lich	Race 7
Race 8	Race 1	Race 2	Race 3	Race 4	Race 5	Race 6	Race 7	Race 7

What each race can make

The monsters fusion can make in each race, in list order, with the fusion level the game compares against the average. This is not always the monster's own level. Vader is race 2 but is not on its list, so fusion never makes it.

A mutation turns the result into the first monster here whose fusion level is at least the hero's level plus 3.

Special fusions

After the Orbs are retrieved, these pairs give a fixed result in either order.

Asura

Kali + Suzaku

Kali + Shiva

Odin

Kali + Zenon

Kali + Jinn

Suzaku + Shiva

Pazuzu

Kali + Wyrm

Suzaku + Zenon

Suzaku + Jinn

Shiva + Zenon

Shiva + Jinn

Loki

Suzaku + Wyrm

Zenon + Wyrm

Jinn + Wyrm

Shiva + Wyrm

Zenon + Jinn

Story fusion

Cerbero and Orthrus become Solion in a story event.